The PHP error_log setting specifies in which file the PHP script errors are logged. This setting is usually specified in the PHP configuration file php.ini which points to a file in which errors of all PHP scripts on that server are logged. It can be modified per directory basis to maintain separate logs for different set of scripts. For users on shared hosting account it is a bonus feature of being able to modify the error_log setting. Normally the error log is a single file for an entire server which is stored in one central place which is only accessible by the root user. But by modifying the error_log setting using htaccess you can have a separate error log for your web hosting account. The php_value error_log requires the php_flag log_errors to be On.
Set PHP error_log value in .htaccess
Go to your document root (www or public_html or htdocs folder) and create a file name.htaccess if it already exists edit it. First the log_errors flag should be turned on then the error_log should be set. The error_log file should be an absolute path e.g. /home/username/php_errors.log. Add the following to your .htaccess file.
php_flag log_errors On
php_value error_log "/home/username/php_errors.log"
It is recommended to have the log file outside the document root so that it is not viewable by the public. Save the file, create a PHP file with errors and test it.