In order to change a php.ini setting you would simply create a php.ini file in your home directory and input the needed variables inside, like below:
In order to change the memory_limit to 128M:
memory_limit = 128M
If you wanted to change upload_max_filesize to 20M you would put:
upload_max_filesize = 20M
Once you have the options you want in the file, you will need to manually edit the .htaccess file in public_html and add the below line to it:
SetENV PHPRC /home/username
Please remember to change "username" to your actual cPanel username!