Self-modifying quasi-quine. Try running it, then piping the output into PHP. Then piping that output into PHP again. Then piping that output into PHP again. Then piping that output into PHP again. Then piping that output into PHP again. Then piping that output into PHP again.

mutation.php

<?php
$data = <<<'DATA'
$program = <<<PROGRAM
<?php
\$data = <<<'DATA'\n$data\nDATA;
$data

PROGRAM;
$n = 0;
if ($n >= 5) {
    echo "OMFG!\n";
} else {
    echo str_replace('$n = 0;', '$n = '.($n+1).';', $program);
}
DATA;
$program = <<<PROGRAM
<?php
\$data = <<<'DATA'\n$data\nDATA;
$data

PROGRAM;
$n = 0;
if ($n >= 5) {
    echo "OMFG!\n";
} else {
    echo str_replace('$n = 0;', '$n = '.($n+1).';', $program);
}

@enygma you got it!