You can do this through JS rather than using PHP as the click is going to happen on the browser. Try the following code
Code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Untitled Document</title>
</head>
<body>
<p><a href="http://www.test.com/Search?Cat=1" id="one">Click to add Parameters</a></p>
<script type="text/javascript">
document.getElementById("one").onclick = function(){
this.href += "&User=1"
;
alert(this.href); //Just to get a notification you can remove this.
}
</script>
</body>
</html>
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