This is not a real function, but a language construct.
PHP >= 4.2.0 does NOT print the status if it is an integer.
The exit() function terminates execution of the script. It prints status just before exiting.
If status is an integer, that value will also be used as the exit status. Exit statuses should be in the range 0 to 254, the exit status 255 is reserved by PHP and shall not be used. The status 0 is used to terminate the program successfully.
$filename = '/path/to/data-file';
$file = fopen($filename, 'r')
or exit("unable to open file ($filename)");
//exit program normally
//exit with an error code
The die() function is an alias for exit().
See also: register_shutdown_function().