que tal caricatos:
  
mira aqui te envio los 2 scripts haber si tu encuentras en donde esta el error: 
archivo javascript
:  
 Código PHP:
    <html>
<head>
<script language="JavaScript">
var x,y;
str = location.search;
                                                                                                                             
if (str != "") {
                                                                                                                             
    commaloc = str.indexOf(",");
                                                                                                                             
    x =  str.substring(1, commaloc);
   document.write("<p>valor de x: "+x);
   y= str.substring(commaloc+1, str.length);
                                                                                                                             
   document.write("<p>valor de y: "+y);
}
                                                                                                                             
</script>
</head>
                                                                                                                             
<body>
<form method = "post" action="prueba.php">
<a href="prueba.php"><img src="laguna_padul2.JPG" ismap ></a>
<input type=hidden name=x value=x>
<input type=hidden name=y value=y>
<input type=submit name=enviar value=enviar>
                                                                                                                             
</form>
</body>
</html> 
   
  archivo prueba.php 
 Código PHP:
    <?php
                                                                                                                             
    $x = $_GET["x"];
    $y = $_GET["y"];
                                                                                                                             
   echo "la coordenada en x vale: ".$x."\n";
   echo "la coordenada en y vale: ".$y."\n";
                                                                                                                             
                                                                                                                             
?>    
  espero me entiendas. 
gracias