PHP reports errors in response to a number of internal error conditions. These may be used to signal a number of different conditions, and can be displayed and/or logged as required.
Every error that PHP generates includes a type. A list of these error types is available, along with a short description of their behaviour and how they can be caused.
If no error handler is set, then PHP will handle any errors that occur
according to its configuration. Which errors are reported and which are
ignored is controlled by the
php.ini directive, or at runtime by calling
error_reporting(). It is strongly recommended that the
configuration directive be set, however, as some errors can occur before
execution of your script begins.
In a development environment, you should always set
E_ALL, as you need to be aware of and fix the
issues raised by PHP. In production, you may wish to set this to a less
verbose level such as
E_ALL & ~E_NOTICE & ~E_DEPRECATED, but
in many cases
E_ALL is also appropriate, as it may
provide early warning of potential issues.
What PHP does with these errors depends on two further php.ini directives.
controls whether the error is shown as part of the script's output. This
should always be disabled in a production environment, as it can include
confidential information such as database passwords, but is often useful to
enable in development, as it ensures immediate reporting of issues.
In addition to displaying errors, PHP can log errors when the
directive is enabled. This will log any errors to the file or syslog
can be extremely useful in a production environment, as you can log errors
that occur and then generate reports based on those errors.
If PHP's default error handling is inadequate, you can also handle many types of error with your own custom error handler by installing it with set_error_handler(). While some error types cannot be handled this way, those that can be handled can then be handled in the way that your script sees fit: for example, this can be used to show a custom error page to the user and then report more directly than via a log, such as by sending an e-mail.