Session Variables Not Being Stored

I have the following PHP code:

session_start(); 
$zip = $_GET['zip'];
$_SESSION['zip'] = $zip;
echo $_SESSION['zip']; //output is 98121, or whatever value I assigned

Then when I go to the next page using the following code:

 <div id="heading"><h2>What day and time would you like your stuff picked up?</h2></div>


<div class="nav_buttons">
            <button class="previous">Previous</button>
            <button class="next">Next</button>
</div><!--end nav_buttons--> 
<form action="/wordpress/junk-details" method="post">
<?php for($j = 0; $j < 7; $j++): ?>
    <div class="date_container" id="date_container_<?php echo $j;?>" >
        <?php for($i = 0; $i < 7; $i++): ?>
            <div class="day">
                <div class="date" id="">

                </div><!--end date--> 
            </div><!--end day--> 
        <?php endfor; ?>

    </div><!--end date_container--> 
<?php endfor; ?>
<div class="nav_buttons">
            <button class="next" id="goto_step2" type="image" value="next step"><span>Next Step: Details</span></button>
</div><!--end nav_buttons--> 
</form>

and I do a var_dump($_SESSION), the value is NULL. I’ve checked my php.ini file, but didn’t see anything out of the ordinary. I’ve searched other similar topics on SO, but none of those have helped.

Here is a list of links I’ve tried:

PHP Session data not being saved

PHP Session not Saving

PHP Session Data Not Being Stored

Here are some details that might help:

I’m using PHP 5.5.3

I’m using a Mac OSX 10.9

I’m using the localhost as a server

This issue is on both Chrome and Firefox browsers

This is a wordpress site

I’m not sure what this could be. My sessions work when I use them on the back of my WordPress plugin