Here is another example that measures time. In contrast to the previous one where participants clicked into the image itself here we use submit buttons.
<HTML>
<?
    $sec=explode(" ",microtime());
    $sec=$sec[0]+$sec[1];
    if (! $SEC)
        echo "<HEAD>
            <META HTTP-EQUIV=Refresh CONTENT=6;URL=$SCRIPT_NAME?SEC=$sec>
            </HEAD>";
    echo "<BODY BGCOLOR=white>
        <FORM METHOD=POST ACTION=$SCRIPT_NAME>\n";
    if (! $SEC) {
    echo "<CENTER><IMG SRC=123.gif></CENTER><BR>
        You have 6 seconds for the next question:<P>
        David Hume was born in?
        <INPUT TYPE=SUBMIT NAME=A VALUE=1711>
        <INPUT TYPE=SUBMIT NAME=A VALUE=1723>
        <INPUT TYPE=HIDDEN NAME=SEC VALUE=$sec>";
    }
    else {
        $diff = $sec-$SEC;
        echo "Answering took you $diff seconds.<BR>";
        switch ($A) {
          case 1711: echo "Correct. David Hume was born 7th May 1711.";break;
          case 1723: echo "Wrong. 1723 is the year when  Adam Smith was baptised.";break;
          default:   echo "You have been too slow...";
        }
    }
?>
</FORM>
</BODY>
</HTML>