add_post_meta does not respect the content

Question

I insert this and it works fine:

add_post_meta( get_the_ID(), 'robots', 'TEXT', true );

After adding that, I check the database and everything is fine, the result is TEXT.

But if I insert this:

add_post_meta( get_the_ID(), 'robots', 'a:2:{i:0;s:7:"noindex";i:1;s:8:"nofollow";}', true );

The result is:

s:43:"a:2:{i:0;s:7:"noindex";i:1;s:8:"nofollow";}";

And I need the result to be exactly the same as I have added it:

a:2:{i:0;s:7:"noindex";i:1;s:8:"nofollow";}

I need not to automatically add that s:43:" and the "; at the end.

Can anybody help me please?

Answer 1

Why the content is not being respected

And it's not just add_post_meta(), but any functions like add_term_meta() and add_user_meta() that uses add_metadata().

And the reason why the value gets serialized again is because add_metadata() will automatically serialize the meta value using maybe_serialize() which (unfortunately) will serialize the value even if it's already serialized.

How to tackle the issue

In your case, if possible, pass an array instead of the serialized value/string:

// A non-scalar value will be automatically serialized by maybe_serialize().
$value = array( 'noindex', 'nofollow' );

add_post_meta( get_the_ID(), 'robots', $value, true );

However, if you retrieved/received the value from somewhere and you need to retain its exact format when saving, then — as you've already figured it out — you'll need to unserialize the value before passing it to add_post_meta() to prevent the double serialization.

But in WordPress, instead of using the native unserialize() function in PHP, I suggest you to use maybe_unserialize() instead:

$value = 'a:2:{i:0;s:7:"noindex";i:1;s:8:"nofollow";}';

// Unserialize the value and pass it to add_post_meta().
add_post_meta( get_the_ID(), 'robots', maybe_unserialize( $value ), true );