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

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

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

<?php
$message = 'hello'
// 没有 "use"
$example = function () {
  var_dump($message);
};
echo $example();
// 继承 $message
$example = function () use($message) {
  var_dump($message);
};
echo $example();
// Inherited variable's value is from when the function
// is defined, not when called
$message = 'world'echo $example();
// Reset message
$message = 'hello'
// 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 = 'world'echo $example();
// Closures can also accept regular arguments
$example = function ($arg) use($message) {
  var_dump($arg . ' ' . $message);
};
$example("hello");
?>
 

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

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

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