PHP/function variable

About

A variable can store a reference to a function, which can then be called from the variable.

Named Functions/Methods

The thing that isn't explained very well in the documentation is that if you want to assign a predefined function to a variable, you just use a string which is a callable/accessible form of the function.

function Example1() { echo "Example1 was called!\n"; }

#$fEx1 = Example1;  // does not work; sees "Example1" as a constant
$fEx1 = 'Example1';

$fEx1();  // prints "Example1 was called!"

class cExample2 {
    static public function Test1() { echo "Test1 was called!\n"; }
}

#$fEx2 = cExample2::Test1;  // does not work; sees "Test1" as a class constant
$fEx2 = 'cExample2::Test1';
$fEx2(); // prints "Test1 was called!"

The variable can be used to call the function in exactly the same way as if it had been assigned an anonymous function.

Anonymous Functions

official documentation

$fEx3 = function() { echo "Anonymous function was called!\n"; };
$fEx3();  // prints "Anonymous function was called!"

Unlike with predefined functions, however, the type for the variable is Callable rather than string.

Functions as Function Parameters

If you declare a function parameter as Callable, that parameter will still accept as string argument, and that argument will remain as a string (i.e. will not be converted to Callable).

The following code, run after the above examples, illustrates this:

function TestFunc2(Callable $f) {
    echo 'Calling the function: ';
    $f();
    echo 'Type for f: '.gettype($f)."\n";
}
TestFunc2($fEx3);
TestFunc2($fEx2);

Output:

Example1 was called!
Test1 was called!
Anonymous function was called!
Calling the function: Anonymous function was called!
Type for f: object
Calling the function: Test1 was called!
Type for f: string

Documentation

• Language Reference:
  • Types:
    • Callbacks / Callables
  • Functions:
    • Anonymous functions
    • Variable functions
    • Function arguments
• Function Reference:
  • Variable and Type Related Extensions:
    • Function Handling:
      • Function handling Functions:
        • call_user_func()