php - wp_session variables being overwritten by $_POST -

i trying post form $wp_session working fine - when perform again overwrites array have put there in first place, need arrays carry on building collection / shopping basket.

my form:

<form class="form1" method="post" action="" id="form1"> <fieldset> <ul>         <label for="name">exercise id</label><span>         <input type="text" name="exerciseid" placeholder="exercise id" class="required" role="input" aria-required="true"/></span>          <label for="name">exercise reps</label><span>         <input type="text" name="sets" placeholder="sets" class="required" role="input" aria-required="true"/></span>          <label for="name">exercise reps</label><span>         <input type="text" name="reps" placeholder="sets" class="required" role="input" aria-required="true"/></span>          <input  class="submit .transparentbutton" value="next" type="submit" name="submit"/>   </ul> <br/> </fieldset> </form> 

and php:

<?php  global $wp_session;  if (isset($_post['submit'])) {  $wp_session['collection'] = array($post['exerciseid'],$_post['sets'],$post['reps']);  }  print_r($wp_session);  ?>  

many in advance.

to have basket, have add products array:

<?php  global $wp_session;  if (isset($_post['submit'])) {  $wp_session['collection'][] = array($_post['exerciseid'],$_post['sets'],$_post['reps']);  }  print_r($wp_session);  ?>  

you may want organize array exerciseid, can add quantity. advice create objects, more flexible arrays