45 个必知必会的 PHP 面试题

青灯夜游

发布时间：2022-09-20 10:45:40

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来源于learnku

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本篇文章给大家总结了45 个必知必会的 php 面试题。有一定的参考价值，有需要的朋友可以参考一下，希望对大家有所帮助。

45 个必知必会的 PHP 面试题

Q1: == 和 === 之间有什么区别？#

话题: PHP困难: ⭐

如果是两个不同的类型，运算符 == 则在两个不同的类型之间进行强制转换
=== 操作符执行’类型安全比较‘

这意味着只有当两个操作数具有相同的类型和相同的值时，它才会返回 TRUE。

1 === 1: true
1 == 1: true
1 === "1": false // 1 是一个整数, "1" 是一个字符串
1 == "1": true // "1" 强制转换为整数，即1
"foo" === "foo": true // 这两个操作数都是字符串，并且具有相同的值

? 源自: stackoverflow.com

Q2: 如何通过引用传递变量？#

话题: PHP困难: ⭐

为了能够通过引用传递变量，我们在其前面使用 &，如下所示：

立即学习“PHP免费学习笔记（深入）”；

$var1 = &$var2

? 源自: guru99.com

Q3: $GLOBAL 是什么意思？#

话题: PHP困难: ⭐

$GLOBALS 是关联数组，包含对脚本全局范围内当前定义的所有变量的引用。

? 源自: guru99.com

Q4: ini_set () 有什么用处？#

话题: PHP困难: ⭐

PHP 允许用户使用 ini_set () 修改 php.ini 中提到的一些设置。此函数需要两个字符串参数。第一个是要修改的设置的名称，第二个是要分配给它的新值。

给定的代码行将启用脚本的 display_error 设置 (如果它被禁用)。

ini_set('display_errors', '1');

我们需要将上面的语句放在脚本的顶部，以便该设置一直保持启用状态，直到最后。此外，通过 ini_set () 设置的值仅适用于当前脚本。此后，PHP 将开始使用 php.ini 中的原始值。

? 源自: github.com/Bootsity

Q5: 我应该在什么时候使用 require 和 include 呢？#

话题: PHP
困难: ⭐⭐

require() 函数与 include() 函数相同，只是它处理错误的方式不同。如果出现错误，include() 函数会生成警告，但脚本会继续执行。require() 函数会产生致命错误，脚本会停止。

我的建议是 99.9% 的时间里只使用 require_once。

使用 require 或 include 代替意味着您的代码在其他地方不可重用，即您引入的脚本实际上是在执行代码，而不是提供类或某些类功能库。

? Source: #0

Q6: PHP 中的 stdClass 是什么？#1

主题: PHP
难度: ⭐⭐

stdClass 只是将其他类型强制转换为对象时使用的通用” 空’’类。stdClass 不是 PHP 中对象的基类。这可以很容易地证明:

class Foo{}
$foo = new Foo();
echo ($foo instanceof stdClass)?'Y':'N'; // 输出'N'

对于匿名对象，动态属性等很有用。

考虑 StdClass 的一种简单使用场景是替代关联数组。请参见下面的示例，该示例显示 json_decode() 如何允许获取 StdClass 实例或关联数组。
同样但未在本示例中显示的 SoapClient::__soapCall 返回一个 StdClass 实例。

//带有StdClass的示例
$json = '{ "foo": "bar", "number": 42 }';
$stdInstance = json_decode($json);

echo $stdInstance - > foo.PHP_EOL; //"bar"
echo $stdInstance - > number.PHP_EOL; //42

//Example with associative array
$array = json_decode($json, true);

echo $array['foo'].PHP_EOL; //"bar"
echo $array['number'].PHP_EOL; //42

? 源自: #2

Q7: PHP 中的 die () 和 exit () 函数有什么不同？#3

话题: PHP
困难: ⭐⭐

没有区别，它们是一样的。选择 die() 而不是 exit() 的唯一好处可能是你节省了额外键入一个字母的时间.

? 源自: #4

Q8: 它们之间的主要区别是什么#5

话题: PHP
困难: ⭐⭐

const 和 define 的根本区别在于，const 在编译时定义常量，而 define 在运行时定义常量。

const FOO = 'BAR';
define('FOO', 'BAR');

// but
if (...) {
    const FOO = 'BAR';    // 无效
}
if (...) {
    define('FOO', 'BAR'); // 有效
}

同样在 PHP 5.3 之前，const 命令不能在全局范围内使用。你只能在类中使用它。当你想要设置与该类相关的某种常量选项或设置时，应使用此选项。或者你可能想要创建某种枚举。一个好的 const 用法的例子是摆脱了魔术数字。

Define 可以用于相同的目的，但只能在全局范围内使用。它应该仅用于影响整个应用程序的全局设置。

除非你需要任何类型的条件或表达式定义，否则请使用 consts 而不是 define()—— 这仅仅是为了可读性！

? 源自: #6

Q9: isset () 和 array_key_exists () 之间有什么区别？#7

话题: PHP
困难: ⭐⭐

array_key_exists 它会告诉你数组中是否存在键，并在 $a 不存在时报错。
如果 key 或变量存在且不是 null，isset 才会返回 true。当 $a 不存在时，isset 不会报错。

考虑:

$a = array('key1' => 'Foo Bar', 'key2' => null);

isset($a['key1']);             // true
array_key_exists('key1', $a);  // true

isset($a['key2']);             // false
array_key_exists('key2', $a);  // true

? 源自: #8

Q10: var_dump () 和 print_r () 有什么不同？#9

话题: PHP
困难: ⭐⭐

var_dump 函数用于显示变量 / 表达式的结构化信息，包括变量类型和变量值。数组递归浏览，缩进值以显示结构。它还显示哪些数组值和对象属性是引用。
print_r() 函数以我们可读的方式显示有关变量的信息。数组值将以键和元素的格式显示。类似的符号用于对象。

考虑:

$obj = (object) array('qualitypoint', 'technologies', 'India');

var_dump($obj) 将在屏幕的输出下方显示:

object(stdClass)#1 (3) {
 [0]=> string(12) "qualitypoint"
 [1]=> string(12) "technologies"
 [2]=> string(5) "India"
}

stdClass Object ( 
 [0] => qualitypoint
 [1] => technologies
 [2] => India
)

? 源自: stackoverflow.com0

Q11: 解释不同的 PHP 错误是什么stackoverflow.com1

话题: PHP
困难: ⭐⭐

notice 不是一个严重的错误，它说明执行过程中出现了一些错误，一些次要的错误，比如一个未定义的变量。
当出现更严重的错误，如 include () 命令引入不存在的文件时，会给出警告 warning。这个错误和上面的错误发生，脚本都将继续。
fatal error 致命错误将终止代码。未能满足 require () 将生成这种类型的错误。

? 源自: stackoverflow.com2

Q12: 如何在 PHP 中启用错误报告？

话题: PHP
困难: ⭐⭐

检查 php.ini 中的 “display_errors” 是否等于 “on”，或者在脚本中声明 “ini_set('display_error',1)”。

然后，在你的代码中包含 “ERROR_REPORTING(E_ALL)”，以便在脚本执行期间显示所有类型的错误消息。
? 源自: stackoverflow.com3

Q13: 使用默认参数声明某些函数

话题: PHP
困难: ⭐⭐

思考:

function showMessage($hello = false){
  echo ($hello) ? 'hello' : 'bye';
}

? 源自: stackoverflow.com3

Q14: PHP 是否支持多重继承？

话题: PHP
困难: ⭐⭐

PHP 只支持单一继承；这意味着使用关键字’extended’只能从一个类扩展一个类。

? 源自: guru99.com

Q15: 在 PHP 中，对象是按值传递还是按引用传递？

话题: PHP
困难: ⭐⭐

在 PHP 中，通过值传递的对象。

? 源自: guru99.com

Q16:$a != $b 和 $a !== $b ，之间有什么区别？

话题: PHP
困难: ⭐⭐

!= 表示 不等于 (如果 $a 不等于 $b，则为 True)， !== 表示 不全等 (如果 $a 与 $b 不相同，则为 True).

? 源自: guru99.com

Q17: 在 PHP 中，什么是 PDO？

话题: PHP
困难: ⭐⭐

PDO 代表 PHP 数据对象。

它是一组 PHP 扩展，提供核心 PDO 类和数据库、特定驱动程序。它提供了供应商中立、轻量级的数据访问抽象层。因此，无论我们使用哪种数据库，发出查询和获取数据的功能都是相同的。它侧重于数据访问抽象，而不是数据库抽象。

? 源自: github.com/Bootsity

Q18: 说明我们如何在 PHP 中处理异常？

Topic: PHP
Difficulty: ⭐⭐

当程序执行出现异常报错时，后面的代码将不会再执行，这时 PHP 将会尝试匹配第一个 catch 块进行异常的处理，如果没有捕捉到异常程序将会报致命错误并显示”Uncaught Exception”。
可以在 PHP 中抛出和捕获异常。

为了处理异常，代码可以被包围在”try” 块中.
每个 try 必须至少有一个对应的 catch 块。多个不同的 catch 块可用于捕获不同类的异常。
在 catch 块中也可以抛出异常（或重新抛出之前的异常）。

思考:

try {
    print "this is our try block n";
    throw new Exception();
} catch (Exception $e) {
    print "something went wrong, caught yah! n";
} finally {
    print "this part is always executed n";
}

? Source: github.com/Bootsity

Q19: 区分 echo 和 print ()

Topic: PHP
Difficulty: ⭐⭐

echo 和 print 基本上是一样的。他们都是用来打印输出数据的。

区别在于:

echo 没有返回值，而 print 的返回值为 1，因此 print 可以在表达式中使用。
echo 可以接受多个参数一起输出 (但是这种多个的输出方式很少见)，而 print 一次只可以输出一个参数。
echo 的输出比 print 效率要高一些 .

? Source: github.com/Bootsity

Q20: require_once 和 require 在什么场景下使用？

Topic: PHP
Difficulty: ⭐⭐⭐

require_once() 作用与 require() 的作用是一样的，都是引用或包含外部的一个 php 文件，require_once() 引入文件时会检查文件是否已包含，如果已包含，不再包含 (require) 它。

我建议在 99.9% 的时候要使用 require_once

使用 require 或 include 意味着您的代码不可在其他地方重用，即您要拉入的脚本实际上是在执行代码，而不是提供类或某些函数库。

? Source: #0

Q21: 判断 PHP 数组是否是关联数组

Topic: PHP
Difficulty: ⭐⭐⭐

思考:

function has_string_keys(array $array) {
  return count(array_filter(array_keys($array), 'is_string')) > 0;
}

如果 $array 至少有一个字符串类型的 key ，它将被视为关联数组。

? Source: #2

Q22: 如何将变量和数据从 PHP 传至 Javascript

Topic: PHP
Difficulty: ⭐⭐⭐

这里有几种实现方法：

使用 Ajax 从服务端获取你需要的数据。

思考 get-data.php:

echo json_encode(42);

思考 index.html:

可以在网页任何地方输出数据，然后使用 JavaScript 从 DOM 中获取信息.

直接在 JavaScript 代码中 echo 数据。

? Source: #3

Q23: 有一个方法可以复制一个 PHP 数组至另一个数组吗？

Topic: PHP
Difficulty: ⭐⭐⭐

PHP 数组通过复制进行赋值，而对象通过引用进行赋值。所有默认情况下，PHP 将复制这个数组。这里有一个 PHP 参考，一目了然：

$a = array(1,2);
$b = $a; // $b 是一个不同的数组
$c = &$a; // $c 是 $a 的引用

? Source: #4

Q24: What will be returned by this code?

Topic: PHP
Difficulty: ⭐⭐⭐

Consider the code:

$a = new stdClass();
$a->foo = "bar";
$b = clone $a;
var_dump($a === $b);

What will be echoed to the console?

Two instances of the same class with equivalent members do NOT match the === operator. So the answer is:

bool(false)

? Source: stackoverflow.com

Q25: What will be returned by this code? Explain the result.

Topic: PHP
Difficulty: ⭐⭐⭐

Consider the code. What will be returned as a result?

$something = 0;
echo ('password123' == $something) ? 'true' : 'false';

The answer is true. You should never use == for string comparison. Even if you are comparing strings to strings, PHP will implicitly cast them to floats and do a numerical comparison if they appear numerical. === is OK.

For example

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'1e3' == '1000' // true

also returns true.

? Source: #6

Q26: What exactly is the the difference between array_map, array_walk and array_filter?

Topic: PHP
Difficulty: ⭐⭐⭐

array_walk takes an array and a function F and modifies it by replacing every element x with F(x).
array_map does the exact same thing except that instead of modifying in-place it will return a new array with the transformed elements.
array_filter with function F, instead of transforming the elements, will remove any elements for which F(x) is not true

? Source: #7

Q27: Explain the difference between exec() vs system() vs passthru()?

Topic: PHP
Difficulty: ⭐⭐⭐

exec() is for calling a system command, and perhaps dealing with the output yourself.
system() is for executing a system command and immediately displaying the output - presumably text.
passthru() is for executing a system command which you wish the raw return from - presumably something binary.

? Source: #8

Q28: How would you create a Singleton class using PHP?

Topic: PHP
Difficulty: ⭐⭐⭐

/**
 * Singleton class
 *
 */
final class UserFactory {
    /**
     * Call this method to get singleton
     *
     * @return UserFactory
     */
    public static
    function Instance() {
        static $inst = null;
        if ($inst === null) {
            $inst = new UserFactory();
        }
        return $inst;
    }

    /**
     * Private ctor so nobody else can instantiate it
     *
     */
    private
    function __construct() {

    }
}

To use:

$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();

But:

$fact = new UserFactory()

Throws an error.

? Source: #9

Q29: What is the difference between PDO’s query() vs execute()?

Topic: PHP
Difficulty: ⭐⭐⭐

query runs a standard SQL statement and requires you to properly escape all data to avoid SQL Injections and other issues.
execute runs a prepared statement which allows you to bind parameters to avoid the need to escape or quote the parameters. execute will also perform better if you are repeating a query multiple times.

Best practice is to stick with prepared statements and execute for increased security. Aside from the escaping on the client-side that it provides, a prepared statement is compiled on the server-side once, and then can be passed different parameters at each execution.

? Source: guru99.com0

Q30: What is use of Null Coalesce Operator?

Topic: PHP
Difficulty: ⭐⭐⭐

Null coalescing operator returns its first operand if it exists and is not NULL. Otherwise it returns its second operand.

Example:

$name = $firstName ?? $username ?? $placeholder ?? "Guest";

? Source: github.com/Bootsity

Q31: Differentiate between exception and error

Topic: PHP
Difficulty: ⭐⭐⭐

Recovering from Error is not possible. The only solution to errors is to terminate the execution. Where as you can recover from Exception by using either try-catch blocks or throwing exception back to caller.
You will not be able to handle the Errors using try-catch blocks. Even if you handle them using try-catch blocks, your application will not recover if they happen. On the other hand, Exceptions can be handled using try-catch blocks and can make program flow normal if they happen.
Exceptions are related to application where as Errors are related to environment in which application is running.

? Source: github.com/Bootsity

Q32: What are the exception class functions?

Topic: PHP
Difficulty: ⭐⭐⭐

There are following functions which can be used from Exception class.

getMessage() − message of exception
getCode() − code of exception
getFile() − source filename
getLine() − source line
getTrace() − n array of the backtrace()
getTraceAsString() − formated string of trace
Exception::__toString gives the string representation of the exception.

? Source: github.com/Bootsity

Q33: Differentiate between parameterised and non parameterised functions

Topic: PHP
Difficulty: ⭐⭐⭐

Non parameterised functions don’t take any parameter at the time of calling.
Parameterised functions take one or more arguments while calling. These are used at run time of the program when output depends on dynamic values given at run time There are two ways to access the parameterised function:
- call by value: (here we pass the value directly )
- call by reference: (here we pass the address location where the value is stored)

? Source: github.com/Bootsity

Q34: Explain function call by reference

Topic: PHP
Difficulty: ⭐⭐⭐

In case of call by reference, actual value is modified if it is modified inside the function. In such case, we need to use & symbol with formal arguments. The & represents reference of the variable.

Example:

function adder(&$str2) {  
    $str2 .= 'Call By Reference';  
}
$str = 'This is ';  
adder($str);  
echo $str;

Output:

This is Call By Reference

? Source: github.com/Bootsity

Q35: Why do we use extract()?

Topic: PHP
Difficulty: ⭐⭐⭐

The extract() function imports variables into the local symbol table from an array.
This function uses array keys as variable names and values as variable values. For each element it will create a variable in the current symbol table.
This function returns the number of variables extracted on success.

Example:

$a = "Original";
$my_array = array("a" => "Cat","b" => "Dog", "c" => "Horse");
extract($my_array);
echo "\$a = $a; \$b = $b; \$c = $c";

Output:

$a = Cat; $b = Dog; $c = Horse

? Source: github.com/Bootsity

Q36: explain what is a closure in PHP and why does it use the “use” identifier?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

Consider this code:

public function getTotal($tax)
{
    $total = 0.00;

    $callback =
        function ($quantity, $product) use ($tax, &$total)
        {
            $pricePerItem = constant(__CLASS__ . "::PRICE_" .
                strtoupper($product));
            $total += ($pricePerItem * $quantity) * ($tax + 1.0);
        };

    array_walk($this->products, $callback);
    return round($total, 2);
}

Could you explain why use it?

This is how PHP expresses a closure. Basically what this means is that you are allowing the anonymous function to “capture” local variables (in this case, $tax and a reference to $total) outside of it scope and preserve their values (or in the case of $total the reference to $total itself) as state within the anonymous function itself.

A closure is a separate namespace, normally, you can not access variables defined outside of this namespace.

use allows you to access (use) the succeeding variables inside the closure.
use is early binding. That means the variable values are COPIED upon DEFINING the closure. So modifying $tax inside the closure has no external effect, unless it is a pointer, like an object is.
You can pass in variables as pointers like in case of &$total. This way, modifying the value of $total DOES HAVE an external effect, the original variable’s value changes.

? Source: guru99.com7

Q37: What exactly are late static bindings in PHP?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

Basically, it boils down to the fact that the self keyword does not follow the same rules of inheritance. self always resolves to the class in which it is used. This means that if you make a method in a parent class and call it from a child class, self will not reference the child as you might expect.

Late static binding introduces a new use for the static keyword, which addresses this particular shortcoming. When you use static, it represents the class where you first use it, ie. it ‘binds’ to the runtime class.

Consider:

class Car {
    public static
    function run() {
        return static::getName();
    }

    private static
    function getName() {
        return 'Car';
    }
}

class Toyota extends Car {
    public static
    function getName() {
        return 'Toyota';
    }
}

echo Car::run(); // Output: Car
echo Toyota::run(); // Output: Toyota

? Source: guru99.com8

Q38: How to measure execution times of PHP scripts?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

I want to know how many milliseconds a PHP while-loop takes to execute. Could you help me?

You can use the microtime function for this.

Consider:

$start = microtime(true);
while (...) {

}
$time_elapsed_secs = microtime(true) - $start;

? Source: guru99.com9

Q39: What is the best method to merge two PHP objects?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

//We have this:
$objectA->a;
$objectA->b;
$objectB->c;
$objectB->d;

//We want the easiest way to get:
$objectC->a;
$objectC->b;
$objectC->c;
$objectC->d;

This works:

$obj_merged = (object) array_merge((array) $obj1, (array) $obj2);

You may also use array_merge_recursive to have a deep copy behavior.

One more way to do that is:

foreach($objectA as $k => $v) $objectB->$k = $v;

This is faster than the first answer in PHP versions = 7 the first answer is something like 400% faster.

? Source: #0

Q40: Compare mysqli or PDO - what are the pros and cons?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

Let’s name some:

PDO is the standard, it’s what most developers will expect to use.
Moving an application from one database to another isn’t very common, but sooner or later you may find yourself working on another project using a different RDBMS. If you’re at home with PDO then there will at least be one thing less to learn at that point.
A really nice thing with PDO is you can fetch the data, injecting it automatically in an object.
PDO has some features that help agains SQL injection
In sense of speed of execution MySQLi wins, but unless you have a good wrapper using MySQLi, its functions dealing with prepared statements are awful. inserts - almost equal, selects - mysqli is2.5% faster for non-prepared statements/6.7% faster for prepared statements.

? Source: #1

Q41: What is use of Spaceship Operator?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

This operator will offer combined comparison in that it will:

Return 0 if values on either side are equal
Return 1 if value on the left is greater
Return -1 if the value on the right is greater

Consider:

//Comparing Integers
echo 1 <= > 1; //outputs 0
echo 3 <= > 4; //outputs -1
echo 4 <= > 3; //outputs 1

//String Comparison

echo "x" <= > "x"; // 0
echo "x" <= > "y"; //-1
echo "y" <= > "x"; //1

? Source: github.com/Bootsity

Q42: Does PHP have threading?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

Standard php does not provide any multithreading but there is an (experimental) extension that actually does - pthreads. The next best thing would be to simply have one script execute another via CLI, but that’s a bit rudimentary. Depending on what you are trying to do and how complex it is, this may or may not be an option.

? Source: github.com/Bootsity

Q43: Is PHP single or multi threaded?

Topic: PHP
Difficulty: ⭐⭐⭐⭐

PHP is not single threaded by nature. It is, however, the case that the most common installation of PHP on unix systems is a single threaded setup, as is the most common Apache installation, and nginx doesn’t have a thread based architecture whatever. In the most common Windows setup and some more advanced unix setups, PHP can and does operate multiple interpreter threads in one process.

PHP as an interpreter had support for multi-threading since the year 2000.

? Source: github.com/Bootsity

Q44: Provide some ways to mimic multiple constructors in PHP

Topic: PHP
Difficulty: ⭐⭐⭐⭐⭐

It’s known you can’t put two __construct functions with unique argument signatures in a PHP class but I’d like to do something like this:

class Student 
{
   protected $id;
   protected $name;
   // etc.

   public function __construct($id){
       $this->id = $id;
      // other members are still uninitialised
   }

   public function __construct($row_from_database){
       $this->id = $row_from_database->id;
       $this->name = $row_from_database->name;
       // etc.
   }
}

What is the best way to achieve this in PHP?

I’d probably do something like this:

class Student
{
    public function __construct() {
        // allocate your stuff
    }

    public static function withID( $id ) {
        $instance = new self();
        $instance->loadByID( $id );
        return $instance;
    }

    public static function withRow( array $row ) {
        $instance = new self();
        $instance->fill( $row );
        return $instance;
    }

    protected function loadByID( $id ) {
        // do query
        $row = my_awesome_db_access_stuff( $id );
        $this->fill( $row );
    }

    protected function fill( array $row ) {
        // fill all properties from array
    }
}

Then if i want a Student where i know the ID:

$student = Student::withID( $id );

Technically you’re not building multiple constructors, just static helper methods, but you get to avoid a lot of spaghetti code in the constructor this way.

Another way is to use the mix of factory and fluent style:

class Student
{
    protected $firstName;
    protected $lastName;
    // etc.

    /**
     * Constructor
     */
    public function __construct() {
        // allocate your stuff
    }

    /**
     * Static constructor / factory
     */
    public static function create() {
        $instance = new self();
        return $instance;
    }

    /**
     * FirstName setter - fluent style
     */
    public function setFirstName( $firstName) {
        $this->firstName = $firstName;
        return $this;
    }

    /**
     * LastName setter - fluent style
     */
    public function setLastName( $lastName) {
        $this->lastName = $lastName;
        return $this;
    }
}

// create instance
$student= Student::create()->setFirstName("John")->setLastName("Doe");

? Source: #5

Q45: How could we implement method overloading in PHP?

Topic: PHP
Difficulty: ⭐⭐⭐⭐⭐

You cannot overload PHP functions. Function signatures are based only on their names and do not include argument lists, so you cannot have two functions with the same name.

You can, however, declare a variadic function that takes in a variable number of arguments. You would use func_num_args() and func_get_arg() to get the arguments passed, and use them normally.

Consider:

function myFunc() {
    for ($i = 0; $i < func_num_args(); $i++) {
        printf("Argument %d: %s\n", $i, func_get_arg($i));
    }
}

/*
Argument 0: a
Argument 1: 2
Argument 2: 3.5
*/
myFunc('a', 2, 3.5);

? Source: github.com/Bootsity

为了处理异常，代码可能被包围在一个 try 块中。

每个 try 必须至少有一个提示。