int filemtime (string filename)
Returns the time the file was last modified, or false in case of an error. The time is returned as a Unix timestamp.
The results of this function are cached. See clearstatcache() for more details.
Note: This function returns the time when the data blocks of a file were being written to, that is, the time when the content of the file was changed. Use date() on the result of this function to get a printable modification date for use in page footers.