onSubmit with PHP doesn't update PHP SESSION details

I am trying to construct a form that takes data from a MySQL database and populates a drop down list. That's all OK. However, what's not happening is that when a user selects a name from the drop down list, the SESSION variable is not getting set. Here's the code from the browser source:
<code>

<script type='text/javascript'>
function OnChange_form1_vendors_name5(x){
var y=document.getElementById(x).value;
document.form2_vendors_name5.elements[0].value=y;
document.form2_vendors_name5.submit();
}
</script>
<form method='POST' action='/eclipse/real_estate/commission_calc.php?action=0&url=' name='form1_vendors_name5'>
<select name='form1_vendors_name5' id='vendors_name' width='' onchange='OnChange_form1_vendors_name5(this.id)'>
<option value=''> </option>
<option value=''></option>
<option value='Martin Cameron'>Martin Cameron</option>
<option value='Martin and Jacky Cameron'>Martin And Jacky Cameron</option>

<option value='asdvasd'>Asdvasd</option>
<option value='jk.hjk.'>Jk.hjk.</option>
<option value='ryuiryuir'>Ryuiryuir</option>
</select>
</form>
<form method='POST' action='/eclipse/real_estate/commission_calc.php?action=0&url=' name='form2_vendors_name5'>
<input type='text' name='form2_vendors_name5_session_details5[]' value='' size='' id=''>
</form>

</code>