function submitform(name, val) {
    var myb = document.getElementById("mybutton");
    myb.value = val;
    myb.setAttribute('name', name)
    document.form1.submit();
}
 

... onclick="submitform('add', '<? echo $add ?>'); return false" ...
 

function submitform(val) {
    var myb = document.getElementById("mybutton");
    myb.value = val;
    myb.setAttribute('name', myb.getAttribute('id'))
    document.form1.submit();
}
 

<script type="text/javascript">
function submitform(name, val) {
    var myb = document.getElementById("mybutton");
    myb.value = val;
    myb.setAttribute("name", name)
    document.form1.submit();
}
</script>

<input id="mybutton" name="add" type="hidden" value=""/>
<input id="mybutton" name="selectall" type="hidden" value=""/>
 
<a href="" name="add" id="add" onclick="submitform('add', '<? echo $add; ?>'); return false;"><img src="img/add_cart.jpg" width="164" height="28" border="0"/></a>
<a href="" name="selectall" id="selectall" onclick="submitform('selectall', '<? echo $selectall; ?>'); return false;"><img src="img/select_all.jpg" width="123" height="28" border="0"/></a>

<input id="mybutton" name="add" type="hidden" value=""/>
<input id="mybutton" name="selectall" type="hidden" value=""/>

<?php
 
var_dump($_GET);
$add = '123';
$selectall = 'qwe';
?>
 
<script type="text/javascript">
function submitform(name, val) {
    var myb = document.getElementById("mybutton");
    myb.value = val;
    myb.setAttribute("name", name)
    document.form1.submit();
}
</script>
 
<form name="form1">
    <input id="mybutton" type="hidden" />
    
    <a href="" name="add" id="add" onclick="submitform('add', '<? echo $add; ?>'); return false;">
        <img src="img/add_cart.jpg" width="164" height="28" border="0"/>
    </a>
    
    <a href="" name="selectall" id="selectall" onclick="submitform('selectall', '<? echo $selectall; ?>'); return false;">
        <img src="img/select_all.jpg" width="123" height="28" border="0"/>
    </a>
    
</form>
 

onclick="submitform(this.id, '<? echo $selectall; ?>');