Why does update_post_meta change the value of my meta key?

When I use the update_post_meta() function to change a value of a key:

• when it's a normal string it works
• but when the string is the same as a JSON file it appends some extra string.

For example, when I save this string:

a:1:{i:1;a:6:{s:5:"index";s:1:"0";s:13:"attachment_id";s:1:"0";s:14:"thumbnail_size";s:0:"";s:4:"name";s:3:"aaa";s:4:"file";s:3:"aaa";s:9:"condition";s:3:"all";}}

It is saved on the wp_postmeta table as:

s:162;"a:1:{i:1;a:6:{s:5:"index";s:1:"0";s:13:"attachment_id";s:1:"0";s:14:"thumbnail_size";s:0:"";s:4:"name";s:3:"aaa";s:4:"file";s:3:"aaa";s:9:"condition";s:3:"all";}}"

When I use a short string there are no problems. How can solve this?

My code:

$edd_files='a:1:{i:1;a:6:{s:5:"index";s:1:"0";s:13:"attachment_id";s:1:"0";s:14:"thumbnail_size";s:0:"";s:4:"name";s:3:"aaa";s:4:"file";s:3:"aaa";s:9:"condition";s:3:"all";}}';

update_post_meta($download_id,'edd_download_files',$edd_files);

The reason why this is happening is because update_post_meta() serializes the value you pass as third parameter (see update_metadata()).

Your $edd_files variable is a serialized array -not a "JSON file"- and update_post_meta() is serializing it again before saving it to the database, hence the reason why your serialized string changed like that.

I don't know why you're assigning $edd_files a serialized string but you can convert it back to an array using the maybe_unserialize() function before saving it as a post meta and then the value will be saved on the database as a serialized string as expected:

$edd_files = 'a:1:{i:1;a:6:{s:5:"index";s:1:"0";s:13:"attachment_id";s:1:"0";s:14:"thumbnail_size";s:0:"";s:4:"name";s:3:"aaa";s:4:"file";s:3:"aaa";s:9:"condition";s:3:"all";}}';

// Convert serialized string back into an array
$edd_files = maybe_unserialize($edd_files);

update_post_meta($download_id, 'edd_download_files', $edd_files);

Result: