If you are looking to center the text, use the following function; I'm not promising perfection...
function imagecenteredstring ( &$img, $font, $xMin, $xMax, $y, $str, $col ) {
$textWidth = imagefontwidth( $font ) * strlen( $str );
$xLoc = ( $xMax - $xMin - $textWidth ) / 2 + $xMin + $font;
imagestring( $img, $font, $xLoc, $y, $str, $col );
}