To allow large file upload in PHP, you need to edit php.ini file and change fllowing settings
memory_limit = 512M
upload_max_filesize = 800M
post_max_size = 800M
max_input_time = 6000
max_execution_time = 600
If you have Dedicated or VPS, you can edit php.ini file using ssh. To find location of this file, upload a phpinfo page to your server, that will display location of php.ini used by your web site.
Some control panel like CPanel have option to edit php.ini settings.