Combo Box value to php variable

[beanymanuk]

Select Size:
<select name=clothessize>
<option name=one value=one>S</option>
<option name=two value=two>M</option>
<option name=three value=three>L</option>
<option name=three value=three>XL</option>
</select>

<input type="button" name="btnAddToCart" value="Add To Cart >" onClick="window.location.href='<?php echo $cart_url; ?>';" class="addToCartButton">

 

How can I get the combo box value into a php variable without refreshing the page so I can pass its value though on onClick event?

 

Thankyou in advance

Pete

[beanymanuk]

More explanation

I want a drop down box to onchange, change the value of the url variable s in the code below which is currently showing "small"

 

<input type="button" class="addToCartButton" 
onclick="window.location.href='cart.php?action=add&p=36&s=small';" 
value="Add To Cart >" name="btnAddToCart">