About the use of OR.

Often see such statements:
$file = fopen ($filename, ' r ') or Die ("Sorry, unable to open: $filename");
Or here is the understanding, because in PHP does not distinguish between the data type, so $file can be int also can bool, so such statements do not complain. But the process may not be clear to some friends.
In most languages, in a statement like BOOL or bool, a value is not judged if the previous value is true. So is this, so if the fopen function executes correctly, it returns an int value greater than 0 (which is actually "true"), and the following statement is not executed. If the fopen function fails, it returns false, and the subsequent expression is judged to be true.
After the result executes die (), the program stops executing regardless of what is returned, and displays the specified error message, which is the purpose of debugging.
That's it. ：）