Definition: Opens a file in PHP to read, write, or rewrite to. It is phrased as fopen ( filename, mode)
Modes:
r - Reading only, beginning of file
r+ - Reading and writing, beginning of file
w - Writing only, beginning of file
w+ - Writing and reading, beginning of file
a - Writing only, end of file
a+ - Writing and reading, end of file
x - Create and open for writing only, beginning of file
x+ - Create and open for reading and writing, beginning of file

If the file does not exist and you use w, w+, a or a+ it will attempt to create the file.

Examples: Some examples of opening different files:
<?php
$a = fopen("/home/files/yourfile.txt", "r");
$b = fopen("yourfile.gif", "w");
$c = fopen("http://www.yoursite.com/", "r");
$d = fopen("ftp://user:password@yoursite.com/yourfile.txt", "w");
?>