Detailed introduction to PHP anonymous functions and precautions

This article mainly introduces relevant information about PHP anonymous functions and precautions. Anonymous functions were introduced in PHP5.3. PHP5.3 not only introduced anonymous functions but also had many better and newer features. Let's take a look at the detailed explanation of PHP anonymous functions and precautions. Friends who need it can refer to

PHP Anonymous Functions and Precautions

PHP5.2 Previous: autoload , PDO and MySQLi, type constraints
PHP5.2: JSON support
PHP5.3: deprecated features, anonymous functions, new magic methods, namespaces, late static binding, Heredoc and Nowdoc, const, Ternary operator, Phar
PHP5.4: Short Open Tag, array abbreviation, Traits, built-in Web server, details modified
PHP5.5: yield, list() for foreach, details modified
PHP5.6: Constant enhancement, variable function parameters, namespace enhancement

Now basically everyone uses PHP5.3 and later versions, but I feel that a common phenomenon is that many new features, after such a long time, are still It is not completely popular and is rarely used in projects.

Look at PHP anonymous functions:

'test' => function(){
    return 'test'
},

The definition of PHP anonymous functions is very simple, which is to assign a value to a variable, but this value is a function.

The above is to use the Yii framework to configure the components file and add a test configuration.

What are PHP anonymous functions?

See the official explanation:

Anonymous functions, also called closures, allow you to temporarily create a function without a specified name. The value most commonly used as a callback function argument. Of course, there are other applications as well.

Anonymous function example:

<?php
echo preg_replace_callback(&#39;~-([a-z])~&#39;, function ($match) {
  return strtoupper($match[1]);
}, &#39;hello-world&#39;);
// 输出 helloWorld
?>

Closure functions can also be used as the value of variables. PHP will automatically convert this expression into an object instance of the built-in class Closure. The method of assigning a closure object to a variable is the same as the syntax of ordinary variable assignment, and a semicolon must be added at the end:

Anonymous function variable assignment example:

<?php
$greet = function($name)
{
  printf("Hello %s\r\n", $name);
};
$greet(&#39;World&#39;);
$greet(&#39;PHP&#39;);
?>

Closures can Inherit variables from the parent scope. Any such variables should be passed in using the use language construct.

Inherit variables from parent scope

<?php
$message = &#39;hello&#39;
// 没有 "use"
$example = function () {
  var_dump($message);
};
echo $example();
// 继承 $message
$example = function () use($message) {
  var_dump($message);
};
echo $example();
// Inherited variable&#39;s value is from when the function
// is defined, not when called
$message = &#39;world&#39;echo $example();
// Reset message
$message = &#39;hello&#39;
// Inherit by-reference
$example = function () use(&$message) {
  var_dump($message);
};
echo $example();
// The changed value in the parent scope
// is reflected inside the function call
$message = &#39;world&#39;echo $example();
// Closures can also accept regular arguments
$example = function ($arg) use($message) {
  var_dump($arg . &#39; &#39; . $message);
};
$example("hello");
?>

Notes on anonymous functions in php

After php5.3, php added the use of anonymous functions. Today, we are using anonymous When an error occurs, you cannot declare and use it like a php function. Look at the code in detail.

$callback=function(){ 
 return "aa"; 
}; 
echo $callback();

The printed code is aa;

Look at the example below:

echo $callback(); 
$callback=function(){ 
 return "aa"; 
};

An error is reported at this time Got it! $callback is undeclared, but no error will be reported when using functions declared by PHP itself!

function callback(){ 
 return "aa"; 
} 
echo callback(); //aa 
  
echo callback(); //aa 
function callback(){ 
 return "aa"; 
}

Both of these are printed aa;

When using anonymous functions, anonymous functions are used as variables and must be declared in advance. The same is true in js!