If it’s not already done in php.ini, error reporting can be set dynamically and should be set to allow most errors to be shown:

Syntax

int error_reporting ([ int $level ] )

Examples

// should always be used prior to 5.4
error_reporting(E_ALL);

// -1 will show every possible error, even when new levels and constants are added 
// in future PHP versions. E_ALL does the same up to 5.4.
error_reporting(-1);

// without notices
error_reporting(E_ALL & ~E_NOTICE);

// only warnings and notices.
// for the sake of example, one shouldn't report only those
error_reporting(E_WARNING | E_NOTICE);

errors will be logged by default by php, normally in a error.log file at the same level than the running script.

in development environment, one can also show them on screen:

ini_set('display_errors', 1);

in production however, one should

ini_set('display_errors', 0);

and show a friendly problem message through the use of an Exception or Error handler.