If you’re talking about a PHP upload page and are using 5.3 or 5.4 you can alter PHP settings by creating a custom phprc.
Do note that PHP is not a robust platform when dealing with large file uploads as the entire file is invariably uploaded into memory before being written to disk. Better solutions are available for uploading files. FTP, CGI scripts, etc. can write data to disk in chunks, avoiding any potential problems caused by a lack of available memory for the domain.