Just add a hidden value as shown below and remove the ‘?req_flag=0’ from the action attribute.
<form method="get" action="send_req.php">
<input type="text" name="f_e_mail_add" value="Enter email of your friend" size="35" />
<input type="hidden" id="req_flag" name="req_flag" value="0" />
<input type="submit" value="Send Request" />
</form>
solved HTML form string error [closed]