PHP stores error logs in /var/log/apache2 if PHP is an apache2 module.
Shared hosts are often storing log files in your root directory /log subfolder.
But…if you have access to a php.ini file you can do this:

error_log = /var/log/php-scripts.log

According to rinogo‘s comment: If you’re using cPanel, the master log file you’re probably looking for is stored (by default) at

/usr/local/apache/logs/error_log

If all else fails you can check the location of the log file using

<?php phpinfo(); ?>