PHP Callback Functions
A callback function (often referred to as just "callback") is a function which is passed as an argument into another function.
Any existing function can be used as a callback function. To use a function as a callback function, pass a string containing the name of the function as the argument of another function.
1. Passing Function as a String
You can pass the name of a function as a string to another function.
function my_callback($item) {
return strlen($item);
}
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map("my_callback", $strings);
print_r($lengths);
// Outputs: Array ( [0] => 5 [1] => 6 [2] => 6 [3] => 7 )2. Anonymous Functions as Callbacks
You can pass an anonymous function (closure) directly.
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map(function($item) { return strlen($item); }, $strings);3. User-Defined Functions with Callbacks
You can define your own functions that accept callbacks. Use the callable type hint to ensure the argument is a function.
function process($str, callable $callback) {
// Call the callback function
echo $callback($str);
}
process("Hello World", function($str) {
return strtoupper($str);
});
// Outputs: HELLO WORLD4. call_user_func
The call_user_func() function is used to call the callback given by the first parameter.
function barber($type) {
echo "You wanted a $type haircut, no problem";
}
call_user_func('barber', "mushroom");