phpBB2 doesn't support php5 official, so php6 is a complete no ;)
phpBB3 support php6 perfect, there are special for php6 checks in common.php

i have the same problem but wit another portal 

i simply disable de code like this

 or remove <?php set_magic_quotes_runtime(0);

  or   disable just add // to the code like this 

//set_magic_quotes_runtime(0);

i use the PHP 6 at amost 1 year and is hard to config but is verry good

still have a lot of bugs but noting that cant be resolve

custume ( WebMaster )

You can easily prevent the error:

<?php
if (!function_exists('set_magic_quotes_runtime')) {
    function set_magic_quotes_runtime($val) {
        if ($val) {
            // throw an error or do some work around 
        }
    }
}
?>

or by checking the PP version before calling the function. There's no real need to keep such a function inside PHP once the feature was removed.

See previous comment.