Description:
------------
mysql_num_rows returns NULL for invalid input data, like Strings/Booleans/Null/etc. 
In regards to the documentation this should be bool(false)

http://php.net/mysql_num_rows
"Return Values
The number of rows in a result set on success or FALSE on failure."

Because it is likely better that invalid input data leads to a different failure handling I propose that you add "or NULL for invalid input data" to the documentation of Return Values to fix this.

Test script:
---------------
<? 
var_dump(mysql_num_rows('test'));
?>

Expected result:
----------------
bool(false)

Actual result:
--------------
Warning: mysql_num_rows() expects parameter 1 to be resource, string given in - on line 2
NULL