Declaration

A declaration is a statement that defines a structure, such as a function, a class, method, property, etc. A declaration must occur before the usage of the declared resource.

PHP requires a declaration for some structures, and allows its omission for others. For example, a class must be declared before usage, while a variable can be used without declaration. Properties are currently being moved to a compulsory declaration.

<?php

// cannot call a function before defining it
foo();

function foo() {}

class x {
     function goo() {
             // This is a warning since PHP 8.2, planned to be a fatal error in PHP 9.0
             $this->undeclaredProperty = 1;

             // $b is undefined. It creates a warning, but the code keeps executing.
             $a = $b + 1;
     }
}

?>

Related : Definition