Fatal error: Call to undefined function: gettextareawidth()

[Battle_Ring]

Fatal error: Call to undefined function: gettextareawidth()
gettign that error when tryign to make a thread

heres line 1 to 21

<?php
error_reporting(7);

$templatesused = "newpost_postpreview,error_nosubject,redirect_post thanks,email_moderator,emailsu bject_moderator,newthread_postpoll,newpost_attachm ent,newpost_disablesmiliesopti on,forumrules,newthread,posticons,posticonbit";
$templatesused.=",vbcode_smilies,vbcode_smiliebit, vbcode_smilies_getmore,vbcode_ buttons,vbcode_sizebits,vbcode_fontbits,vbcode_col orbits";
require("./global.php");

// get decent textarea size for user's browser
$textareacols = gettextareawidth();

$action = trim($action);
if (!isset($action) or $action=="") {
$action="newthread";
}

unset ($postpreview);
unset ($parseurlchecked);
unset ($emailchecked);
unset ($disablesmilieschecked);
unset ($signaturechecked);
unset ($previewchecked);