Unnoticed Things about PHP Exception & Error

Do you know:

• What's Throwable, what's the difference between Throwable and Exception
• Can try-catch catch all exceptions and errors in php?
• What kind of error or exception can't be catched by try-catch? And how to deal with these errors?
• Can set_error_handler() handle all errors?
• How many types of error in PHP? And what's the difference between them?

Throwable

Since PHP7, Error class was introduced for fatal but recoverable errors, it could be caught like any other exception, but it does not extend Exception class. To unites these two classes, Error and Exception both implement Throwable interface.
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Can try-catch catch all exceptions and errors in php?

Of course no. The exceptions and errors that work in try-catch would not be beyond Error and Exception categories. And as above explains, Error is only thrown when fatal but recoverable errors happen, so warning error and notice error won't work in try-catch.

As following code shows, we call on an undefined function, which would cause an fatal error, throw an Error. Cause Error is not a subclass of Exception, but it implements Throwable, so it would be caught by catch(Throwable $e) not catch (Exception $e).

try {
    $var = 1;
    $var->method();
} catch (Exception $e) {
      echo "Exception type: " . get_class($e) . "\n";
      echo 'Exception message: ' . $e->getMessage() . "\n";
} catch (Throwable $e) {
    echo "Error type: " . get_class($e) . "\n";
    echo 'Error message: ' . $e->getMessage() . "\n";
}

But what if we just call on an undefined variable, which throws an notice error?

try {
    $var = 1;
    echo $a;
} catch (Exception $e) {
      echo "Exception type: " . get_class($e) . "\n";
      echo 'Exception message: ' . $e->getMessage() . "\n";
} catch (Throwable $e) {
    echo "Error type: " . get_class($e) . "\n";
    echo 'Error message: ' . $e->getMessage() . "\n";
}

Execute above code, the notice error is thrown and not caught, so try-catch does not work for notice error.

Can set_error_handler() handle all errors?

Of course no. set_error_handler() is used to set user defined error handler function, which would work for trigger_error. But the thing we don't notice is that it works for warning error and notice error as well, and error_reporting() has no effect for it. According to PHP manual, set_error_handler() cannot handle E_ERROR, E_PARSE, E_CORE_ERROR, E_CORE_WARNING, E_COMPILE_ERROR, E_COMPILE_WARNING.

error_reporting(7); // error_reporting() won't work while set_error_handler is set
set_error_handler(function ($level, $message, $file = '', $line = 0, $context = []) {
    echo "level: $level\n";
    echo "msg: $message\n";
    echo "file: $file\n";
    echo "line: $line\n";
});

$arr = ['b' => 'c'];
$arr['a'];

As above example shows, we call on an undefined variable, which causes notice error, and this error would be caught by set_error_handler(). Although we set error_reporting(7), which means the level beyond 7 won't be reported, set_error_handler() disable it. Level of notice error reporting is 8, without set_error_handler, the notice error won't be thrown.

How many types of error in PHP?

Basically, four types. Notice error, warning error, parse error and fatal error. The first two can't be dealt with try-catch, not included in Error, can be handled by set_error_handler(). On the contrary, some of the last two can be handled by try-catch, and not by set_error_handler().
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