You can call a function onclick by using AJAX, but it gets kind of complicated. I would take tr8er8's suggestion and just make them buttons. Check out this simple example.
<html>
<body>
<form name="form" method="post"> <input type="submit" name="button1" value="Button 1" />
<input type="submit" name="button2" value="Button 2" />
<input type="submit" name="button3" value="Button 3" />
<input type="submit" name="button4" value="Button 4" />
</form>
</body>
<?php
if(isset($_POST['button1'])){
echo("You clicked button one!");
}//if isset
if(isset($_POST['button2'])){
echo("You clicked button two!");
}//if isset
if(isset($_POST['button3'])){
echo("You clicked button three!");
}//if isset
if(isset($_POST['button4'])){
echo("You clicked button four!");
}//if isset
?>
</html>
