<?php

function get_NMEA_checksum($sentence) {
        // trim beginning of $sentance
        if (strpos($sentence, "$") !== false) {
                $sentence = substr($sentence, strpos($sentence, "$") + 1);
        }
        // trim end of $sentance
        if (strpos($sentence, "*") !== false) {
                $sentence = substr($sentence, 0, strpos($sentence, "*"));
        }

		// loop through characters to get a checksum
        $checksum = false;
        if (strlen($sentence) > 0) {
                $sentance_array = str_split($sentence);
                foreach($sentance_array as $key=>$letter) {
                        // Is this the first value for the checksum?
                        If ($checksum === false) {
                                // Yes. Set the checksum to the value
                                $checksum = $letter;
                        } Else {
                                // No. XOR the checksum with this character's value
                                $checksum = $checksum ^ $letter;
                        }
                }
                // return checksum as hexdecimal string
                return sprintf('%02X', ord($checksum));
        } 
     

        // return boolean FALSE if no characters to evaluate
        return false;
}


// example useage

echo get_NMEA_checksum($_GET['q']);

?>