A PHP array or object or other complex data structure cannot be transported or stored or otherwise used outside of a running PHP script. If you want to persist such a complex data structure beyond a single run of a script, you need to serialize it. That just means to put the structure into a "lower common denominator" that can be handled by things other than PHP, like databases, text files, sockets. The standard PHP function serialize is just a format to express such a thing, it serializes a data structure into a string representation that's unique to PHP and can be reversed into a PHP object using unserialize. There are many other formats though, like JSON or XML.
Take for example this common problem:
How do I pass a PHP array to Javascript?
PHP and Javascript can only communicate via strings. You can pass the string "foo" very easily to Javascript. You can pass the number 1 very easily to Javascript. You can pass the boolean values true and false easily to Javascript. But how do you pass this array to Javascript?
Array ( [1] => elem 1 [2] => elem 2 [3] => elem 3 )The answer is serialization. In case of PHP/Javascript, JSON is actually the better serialization format:
{ 1 : 'elem 1', 2 : 'elem 2', 3 : 'elem 3' }Javascript can easily reverse this into an actual Javascript array.
This is just as valid a representation of the same data structure though:
a:3:{i:1;s:6:"elem 1";i:2;s:6:"elem 2";i:3;s:7:" elem 3";}But pretty much only PHP uses it, there's little support for this format anywhere else.
This is very common and well supported as well though:
<array>
<element key='1'>elem 1</element>
<element key='2'>elem 2</element>
<element key='3'>elem 3</element>
</array>There are many situations where you need to pass complex data structures around as strings. Serialization, representing arbitrary data structures as strings, solves how to do this.