The book I read to research this post was PHP Fast & Easy Development by Julie C Meloni which is an excellent book although it is a little old and I bought it from a car boot sale. One thing I particularly like about this book is it sets out the steps to setting up apache, php and mysql in an easy to follow format.
Here are some uses for PHP:
It performs system functions ie creating directories and modifying permissions.
It gathers data from forms.
It accesses databases and modifies elements of a database.
It sets and accesses cookies.
It starts sessions and uses session objects.
It restricts access to parts of your website.
It creates images.
And it encrypts data.