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I created a custom key and wanted to store multiple values in the key with an array in my plugin using a drop down box, however, I printed out the array and came back out with this value...

Array ( [0] => a:2:{i:0;s:2:"75";i:1;s:2:"68";} )

My thinking was that it would just add another value [1] => 'content', [2] => 'lkajsdf'. That is how I wanted it to happen, but instead I get the result above. It is storing the two ID values that I selected 75 and 68.

So, what does that result mean?

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You're looking at a serialized representation of the array Array( '75', '68' ). Serialization is the process by which PHP stores a data object as a string, much like the manner in which JSON is a string representation of a Javascript object. PHP data structures may be converted into a serialized format via PHP's serialize(), and back again using unserialize().

WordPress also provides functions which perform the necessary action only when needed in order to prevent accidental double-serialization or unserialization: maybe_serialize() and maybe_unserialize(). You can also check yourself with WordPress' is_serialized().

From the comments on PHP's serialize(), the anatomy of a serialized data object is as follows:

String s:size:value;

Integer i:value;

Boolean b:value; (does not store "true" or "false", does store 1 or 0)

Null N;

Array a:size:{key definition;value definition;(repeated per element)}

Object O:strlen(object name):object name:object

size s:strlen(property name):property name:property definition;(repeated per property)

String values are always in double quotes.

Array keys are always integers or strings; using other types as keys produces undesirable results:

  • null => 'value' equates to 's:0:"";s:5:"value";'
  • true => 'value' equates to 'i:1;s:5:"value";'
  • false => 'value' equates to 'i:0;s:5:"value";'
  • array(whatever the contents) => 'value' equates to an "illegal offset type" warning because you can't use an array as a key; however, if you use a variable containing an array as a key, it will equate to 's:5:"Array";s:5:"value";', and attempting to use an object as a key will result in the same behavior as using an array will.

Therefore, we can interpret your particular serialized array a:2:{i:0;s:2:"75";i:1;s:2:"68";} as such:

  • a:2:{ an array of length 2, containing:
    • i:0; at the integer key 0 (i.e. index 0):
      • s:2:"75"; a string of length 2 with the value "75"
    • i:1; at the integer key 1 (i.e. index 1):
      • s:2:"68" a string of length 2 with the value "68"
  • } end of the array

To alternately reflect the values of your items 'content' and 'lkajsdf' rather than their numerical identifiers, then, the array Array( 'content', 'lkajsdf' ) would be serialized as

a:2:{i:0;s:7:"content";i:1;s:7:"lkajsdf";}

Knowing this can come in handy if you ever need to alter some of WordPress's settings, or selectively activate/deactivate specific plugins directly from the database.

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  • We've all run into the question at one point or another ;) . I think perhaps the most important thing to keep in mind with WordPress and serialization is that WordPress stores some data in the database as a serialized string. If you ever go to perform a search-and-replace operation on your WordPress database, think twice, because it may well invalidate any serialized data containing that string. There are a few serialization-aware database search-and-replace tools for that reason.
    – bosco
    May 5, 2015 at 18:25
  • 2
    +1 good overview and I like how you break up the serialized string with comments, similar to how regexp patterns are often explained.
    – birgire
    May 5, 2015 at 20:07
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It seems like the your multiple values were simply serialized by PHP's serialize() function.

You can unserialize it with unserialize().

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After unserializing the result is:

Array
(
   [0] => 75
   [1] => 68
)

You can find the format of PHP serialized array here: Serialized PHP string

a:2 is object array here 's':<i> is the string where i is the length of string

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