javascript - How to get the after question mark value PHP echo -

i need value of add_cart after question mark?

echo "<a  href='index.php?add_cart=<?php echo $pro_id; ?>' name='shto' class='sp' '><button style='float:right;'>cart</button></a>  "; 

something

$value = add_cart; 

try like:

$value = 4544545; //the item number echo "<a  href='index.php?add_cart=$value' name='shto' class='sp' '><button style='float:right;'>cart</button></a>  "; 

and in index.php value with:

$valuefromaddcart = $_get['add_cart'];