<?php
 
if(eregi("^[[:alnum:]]+$",$t1)) {
  printf("alnum<br>");
} else {
  printf("not alnum<br>");
}
 
?>

when invoked with $t1=<some string in cyrillic koi8-r encoding> (example - "????") and correct locale settings (see below) prints "not alnum"

while at the command line grep -E with the same input matches:

    -------
mbravo@tag-ltd:pages$ grep -E '[[:alnum:]]+'
????
????
    -------


PHP configured as - 

'./configure' '--prefix=/opt/www' '--with-apxs=/opt/www/bin/apxs'
'--with-mod_charset' '--with-mysql=/opt' '--with-pgsql=/opt'

posix
Revision                                            $Revision: 1.17 $

LANG  ru_RU.KOI8-R
HTTP_ENV_VARS["LANG"] ru_RU.KOI8-R