$_POST array is sometimes empty

Question

When submitting my form, sometimes the $_POST array is empty and other times it's not (when inputting the exact same thing on the exact same input). My post_max_size is large 500M and I'm only inputting values from checkboxes. What could be causing this? I might have thought it was because of a for loop I'm using, but the $_POST array is fine half the time and empty the other.

 <form action="send.php" method="post" enctype="multipart/form-data">
    <input type="checkbox" name="Facebook" value="Facebook" id="Facebook"/>
    <input type="checkbox" name="Twitter" value="Twitter" id="Twitter"/>
    <?php
    for($i = 0; $i < $blogcount; $i = $i + 1){
    echo '<input type="checkbox" name="Tumblr[]" value="'.$bloglist[$i].'" id="Tumblr'.$bloglist[$i].'"/>';
    } 
    ?>
    <input id="myImage" type="file" accept="image/*" name="file" class="upload" />
    <input type="submit">
</form>

Answer 1

If you don't check any checkbox, it won't post and thus it would give you empty $_POST. All you have to code is isset($_POST['checkboxname'])

Answer 2

Try this

<?php
    for($i = 0; $i < $blogcount; $i = $i + 1){
        ?>
        <input type="checkbox" name="Tumblr[]" value="<?php echo $bloglist[$i] ?>" id="<?php echo 'Tumblr'.$bloglist[$i] ?>"/>
    <?php
    } 
?>

Answer 3

I think you have add value to your submit button to work correctly.

<input type="submit" value="submit" name="submit">