Although most web servers have an inbuilt error log system these access and error logs do not always provide sufficient information on the source of specific PHP errors.
PHP on itself however has an inbuilt error report/handling option.

Here is how to activate this feature in four steps:

01: Edit File php.ini:

sudo nano /etc/php7/apache2/php.ini

set values corresponding to these (line numbers may be different!):

445:    error_reporting = E_ALL & ~E_DEPRECATED & ~E_STRICT
462:    display_errors = On
483:    log_errors = On
571:    error_log = /var/log/php-errors.log

02: Now create the log-file:

sudo touch /var/log/php-errors.log

03: Change ownership to make the file accessible to the Apache user:

sudo chown wwwrun /var/log/php-errors.log

04: Restart Apache:

sudo service apache2 restart

or

sudo systemctl restart apache2

As of now any php-errors will be logged in this file!