New and having a problem I can't find by search

[PanamaExpat]

Hello :

I am trying o set up roundcube on a CentOs 5.9 server running postfix and dovecot. Post fix and dovecot are working just fine but every time I attempt to use either setup or a manually configured roundcube I get this error in my browser:

Fatal error: Call to undefined function filter_var() in /var/www/html/roundcubemail/program/lib/Roundcube/bootstrap.php on line 48

I have looked at the bootstrap php file and as I am not familiar enough with the layout of the software yet I have no idea where to look.

Here are lines 47 through 54 of the bootstrap file:

foreach ($config as $optname => $optval) {
    $ini_optval = filter_var(ini_get($optname), FILTER_VALIDATE_BOOLEAN);
    if ($optval != $ini_optval && @ini_set($optname, $optval) === false) {
        $error = "ERROR: Wrong '$optname' option value and it wasn't possible to set it to required value ($optval).\n"
            . "Check your PHP configuration (including php_admin_flag).";
        if (defined('STDERR')) fwrite(STDERR, $error); else echo $error;
        exit(1);
    }

Has anyone an idea as to what I am missing? I am hoping to use roundcube as a replacement for my 8 years old and still running qmail with squirrelmail server. Need a fresher look and something that scales for mobile clients. Also my old hardware is getting a bit on the "Will it keep running another week?" part of the brain.

The reason I am running CentOS 5.9 is it is on an older but still running IBM e-server x-series 305 that only has 512 megs of RAM. But this is a development box so please bear with me.

[PanamaExpat]

Sorry to have taken your time. I found a clue in another forum and updated PHP from 5.1 to 5.4 and that seems to have solved it... Or at least that part of my problem. I'll post my progress.