Implementing php validation form into front-page.php wordpress -

i got form here trying validate on front page php file wordpress site, whenever click submit takes me post "page not found", why leaving page? not sure wrong code, thinking submit button must not right, shouldn't leave page.

maybe i'm not implementing php code right way. html form @ beginning , php code @ end.

<form id="contact-form"  method="post">                     <ul>                         <li class="desc">email</li>                              <li><input placeholder="i.e.email@email.com" type="" name="email" id="form_email">                             <span><?php echo $email_error ?></span></li>                          <li class="desc">first name</li>                              <li><input type="text" name="firstname" id="form_firstname" ><span><?php echo $firstname_error ?></span></li>                          <li class="desc" >last name</li>                             <li><input type="text" name="lastname" id="form_lastname"><span><?php echo $lastname_error ?></span></li>                          <li class="desc">company name</li>                             <li><input type="text" name="companyname" id="form_companyname"><span><?php echo $companyname_error ?></span></li>                          <li class="desc">phone</li>                             <li><input type="integer" name="phone" id="form_phone"><span><?php echo $phone_error ?></span></li>                          <li><button name="name" value="submit" type="submit"></button><!--<a href="" id="sub"><img src="images/downloadbutton.png"><input type="hidden" name="submit" value="submit"/></a>--></li>                      </ul>                 </form>                 <?php             if(!empty($_post['email']) && !empty($_post['firstname']) && !empty($_post['lastname']) && !empty($_post['companyname']) && !empty($_post['phone'])) { $email = $_post['email']; $firstname = $_post['firstname']; $lastname = $_post['lastname']; $companyname = $_post['companyname']; $phone = $_post['phone'];  if(strlen($email)<5) {     $email_error = '*'; } elseif (strlen($firstname)<1) {     $firstname_error = '*'; } elseif (strlen($lastname)<1) {     $lastname_error = '*'; } elseif (strlen($companyname)<1) {     $companyname_error = '*'; } elseif (strlen($phone)<10) {     $phone_error = '*'; } else{     echo " <ul>             <li>email           : $email </li>             <li>first name      : $firstname</li>             <li>last name       : $lastname</li>             <li>company name    : $companyname</li>             <li>phone           : $phone</li>           </ul>           "; } }  else{         $email_error = '*';         $firstname_error = '*';         $lastname_error = '*';         $companyname_error = '*';         $phone_error = '*'; } ?> 

you can try adding form action

action='<?= get_permalink(page_id); ?>' 

page_id = page process form , may same page form displaying.

thnx