private static vars

example.php

<?php

class Example {
    private static $a = array(1, 2);
    private static $b = null;

    public static function init()
    {
        if (self::$b === null) {
            self::$b = self::$a[0];
        }
    }
    
    public static function yourFunction()
    {
        self::init();
        
        return self::$b;
    }
}