Strings in PHP could be thought of as sentences such as: "I went to the store". Strings differ from variables in that they are a COLLECTION of values in one variable.
Example:
$variable = '9';
$string = 'abcdefghijklmnopqrstuvwxyz';
Stings can be used to store names, tiles, addresses, and all kinds of things. So expect to use them often! Just like variables - you don't need to tell the computer that it is a "string". The computer can tell just by what value you put in it.
$1 = 0; //var
$2 = 'Hi'; //string
$3 = "Fast Monkey"; //string
$4 = 'K'; //var
Now, what you can DO with strings is the cool part! (For a list of all string functions click here.)
Some of the more important string functions are:
The strlen(); Function.
The explode(); Function.
The str_replace(); Function.
The str_split(); Function.
The strip_tags(); Function.
I highly recomend you read Codewalkers String Primer.
Here are some links to help you learn more about strings:
tizag.com - Strings
Zend.com - Adv. Strings
PHP - Strings Type