update_post_meta not saving when value is zero

Question

I have a problem with the update_post_meta function.
I have a user submitted value, which I pass via $_POST and then saving to post meta.

All is working fine, but when the value is '0' the post meta is not updated.

My code:

$id = absint($_POST['ex_id']);
$key = (string) $_POST['id'];
$value = (string) $_POST['value'];

echo update_post_meta($id, 'wpptabs_' . $key, $value);

Does anyone have any idea what might be wrong?

Answer 1

update_post_meta() calls update_metadata which begins with this line:

if ( !$meta_type || !$meta_key )
    return false;

I wrote a simple test (with a lot of cut & paste) to see how string values of 0 are evaluated (instead of looking it up in the PHP manual):

<?php

$meta_key = (string) 0;

if ( ! $meta_key )
    echo "0 exits function<br />";

else {
    echo "0 remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = (string) 0;

if ( ! $meta_key )
    echo "(string) 0 exits function<br />";

else {
    echo "(string) 0 remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = (string) '0';

if ( ! $meta_key )
    echo "(string) '0' exits function<br />";

else {
    echo "(string) '0' remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = 0x0;

if ( ! $meta_key )
    echo "0x0 exits function<br />";

else {
    echo "0x0 remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = (string) 0x0;

if ( ! $meta_key )
    echo "(string) 0x0 exits function<br />";

else {
    echo "(string) 0x0 remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = '0x0';

if ( ! $meta_key )
    echo "'0x0' exits function<br />";

else {
    echo "'0x0' remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

echo '<br />';
$meta_key = (string) '0x0';

if ( ! $meta_key )
    echo "(string) '0x0' exits function<br />";

else {
    echo "(string) '0x0' remains in function<br />";
    echo "&nbsp;&nbsp;\$meta_key = $meta_key<br />";
    echo "&nbsp;&nbsp;(integer) \$meta_key = " . (integer) $meta_key . '<br />';
}

The result was:

0 exits function

(string) 0 exits function

(string) '0' exits function

0x0 exits function

(string) 0x0 exits function

'0x0' remains in function
  $meta_key = 0x0
  (integer) $meta_key = 0

(string) '0x0' remains in function
  $meta_key = 0x0
  (integer) $meta_key = 0

So, to add a value of 0 you could change it to the string '0x0' and cast it to integer when retrieving it. 0x0 is binary (I think) for 0.

Answer 2

PHP treats 0 as equivalent to false for comparisons. Because the update_post_meta function checks for a value before saving, it's exiting because 0 == false.

You could do something like this before saving the post meta, it's a bit hacky but it would work:

if ( 0 === $value ) {
    $value = 'zero';
}

Then when you're retrieving your values later, you just do the opposite:

$value = get_post_meta( $post_id, 'meta-key', true );
if ( 'zero' == $value ) {
    $value = 0;
}

Answer 3

The issue is to do with this part of update_metadata() function within meta.php that update_post_meta() calls

$result = $wpdb->update( $table, $data, $where );
if ( ! $result )
    return false;

As the return value is 0 then the if statement correctly treats it as false, albeit 0 has been saved to the database.

This only happens when it updates. If a new post meta is added with a value of 0 then it returns the line id as expected.