PHP: if file doesn't exist then exit script

Question

<?php
$filename = '/path/to/foo.txt';

if (file_exists($filename)) {
    echo "The file $filename exists";
}
?>

That script only checks if a file exists, but how do I do something like this?

<?php
$filename = '/path/to/foo.txt';

if (file doesnt exist) {
    exit("Your file doesn't exist");
}
?>

Answer 1

As John Conde said, you should negate the file_exists function:

if (!file_exists($filename)) {
    exit("Your file doesn't exist");
}