The PHP static keyword is used to declare properties and methods of a class as static (can be used without creating an instance of the class).

The static keyword is also used to declare variables in a function which keep their value after the function has ended.

<?php
class my_class {
    public static $str = 'Hello World';

    public static function hello() {
        echo my_class::$str;
    }
}

echo my_class::$str;
echo '<br>';
echo my_class::hello();
?>