There are many functions in PHP. You can visit php.net and checkout the documentation.
In this tutorial I will use some functions you will have to use really often.
One important part are the string functions. These are functions to manipulate strings.
$input = 'Hello World';
// get the length of a string
strlen($input); // 11
// replace strings
str_replace('World', 'Toby', $input); // Hello Toby
// get a part of a string
substr($input, 0, 5); // Hello