ref https://www.youtube.com/watch?v=qaOdWRrp-K8

geomag.php

<?php

	$port = STDIN;
	$out = array();

	$b=0;
	$dlc = -1;
    $buffer = array();
    while(!feof($port)) {
        $curByte = ord(fread($port, 1));
		//echo '<< 0x'.dechex($curByte)."\n";

        $buffer[] = $curByte;

        while(count($buffer) > 0 && $buffer[0] != 0xAA) {
            array_shift($buffer);
        }

        if(count($buffer) < 2) continue;

		if($dlc == -1) $dlc = $buffer[1];

		if(count($buffer) < ($dlc+3)) continue;

        switch($buffer[2]) {
			case 0x03:	//Status Report
				if($buffer[1] != 8) {
					echo 'Status report with DLC != 8 - no idea what to do…'."\n";
				}

				echo 'Unique sensor ID is: 0x'.dechex($buffer[5]).dechex($buffer[4]).dechex($buffer[3])."\n";

				echo 'Status: 0x'.dechex($buffer[6]).' (';
				switch($buffer[6]) {
					case 0x00: echo 'No Car detected'; break;
					case 0x01: echo 'Car detected'; break;
					case 0x02: echo 'Not calibrated'; break;
					default: echo 'unknown';
				}
				echo ")\n";

				$voltage = $buffer[7] / 10;
				echo 'Supply voltage: (0x'.dechex($buffer[7]).') -> '.$voltage."V\n";

				$field = (($buffer[9] << 8) | $buffer[8]);
				echo 'Magnetic field: (0x'.dechex($buffer[9]).'-'.dechex($buffer[8]).') -> '.(int)$field."\n";

				echo 'Checksum: 0x'.dechex($buffer[10]).' - ';
				$chk = 0xBB;
				for($i=0; $i<($dlc+2); $i++) $chk ^= $buffer[$i];
				if($chk == $buffer[10]) {
					echo 'OK';
				}else{
					echo '!= 0x'.dechex($chk).' ERR!';
				}
				echo "\n";
				for($i=0; $i<$dlc+3; $i++) array_shift($buffer);
			break;
		}
	}