Randomize your PHP unit tests order by adding a shuffle.

AllTests.php

<?php

/**
* @author
*/

require_once 'Config.php';

class AllTests {
    public static function suite() {
        $suite = new PHPUnit_Framework_TestSuite('PHPUnit');

        $tests = array(
                'FirstFileWithTest',
                'SecondFileWithTest',
                'ThirdFileWithTest',
                'AndSoOnTest'
        );

        // randomize test order
        shuffle($tests);

        foreach ($tests as $key => $value) {
                echo "[$key] $value \n";
                require_once $value . '.php';
                $suite->addTestSuite($value);
        }
        return $suite;
    }
}