Review: Function that measures the width of a text string in pixels.

I have created a function that will measure the width of a text string in pixels. It works ok. If you have optimizations, please let me know. Thanks, Jacob function txt_width ($txt) { $width = 0; $txt_len = strlen($txt); for ($n = 1; $n <= $txt_len; $n++) { switch (substr($txt, $n, 1)) { case 'l'; case '.'; case ' '; case '-'; case 't'; $width += 3; break; default : $width += 8; break; } } return $width; }