PHP Exception Handling – AssertionError

Today we’ll continue with our exploratory PHP Exception Handling series by diving deeper into the PHP AssertionError. As you can probably guess, an AssertionError is (sometimes) thrown when an assert() call fails. However, a number of default settings and configuration differences based on PHP versions means that AssertionErrors may not always behave as you expect them to.

Let’s jump right in by first looking where the AssertionError fits in the overall PHP Exception Hierarchy, then we’ll take a look at some functional code samples to help illustrate how different PHP versions and configurations may alter your own experiences. Let’s get started!

The Technical Rundown

  • All PHP errors implement the Throwable interface, or are extended from another inherited class therein.
  • Error implements the Throwable interface.
  • AssertionError extends the Error class.

When Should You Use It?

Let’s begin by jumping right into some code to test out some assertions. Normally we’d begin with the entire code sample at once so it’s easier to copy for your own experimentation, but in this case we’ll need to check (and change) a number of PHP configuration settings throughout our tests, so we’ll be using the same functions multiple times for that.

For those unaware, the assert() function accepts up to two arguments, but the single required argument should be a testable condition, either in the form of a string or a boolean condition, that will be tested by the PHP engine. If the assertion succeeds, nothing happens. However, if the condition fails PHP will perform one of a handful of (possible) actions, depending on settings, PHP version, and the like. For our purposes, our goal is to have a failed assert() call throw an AssertionError at us.

Additionally, it’s worth noting that this example code is running on a default PHP 7 installation. We’re also using a simple Logging class to help with output, which can be found below:

<!--?php require('kint.php'); /** * Provides basic logging/output functionality. */ class Logging { /** * Logs the passed object, string, or Throwable instance to the console. * * @param $a Primary message or value to be logged. * @param null $b Secondary value, such as boolean for Throwables indicating if error was expected. */ public static function Log($a, $b = null) { if (is_string($a)) { Logging::LogString($a); } elseif ($a instanceof Throwable) { Logging::LogThrowable($a, is_null($b) ? true : $b); } else { Logging::LogObject($a); } } /** * Logs the passed object. * * @param $object Object to be logged. * * @see https://github.com/kint-php/kint Kint tool used for structured outputs. */ private static function LogObject($object) { Kint::dump($object); } /** * Logs the passed string value. * * @param $value Value to be logged. */ private static function LogString($value) { print_r("{$value}\n"); } /** * Logs the passed Throwable object. * Includes message, className if error was expected, and stack trace. * * Uses internal Reflection to retrieve protected/private properties. * * @param $throwable Throwable object to be output. * @param bool $expected Indicates if error was expected or not. */ private static function LogThrowable($throwable, bool $expected = true) { $expected = $expected ? "EXPECTED" : "UNEXPECTED"; $message = substr($throwable->xdebug_message, 1);<br ?--> // Output whether error was expected or not, the class name, the message, and stack trace.
print_r("[{$expected}] {$message}\n");
// Add line separator to keep it tidy.
self::LineSeparator();
}

/**
* Outputs a separator line to log.
*
* @param int $length Length of the line separator.
* @param string $character Character to use as separator.
*/
public static function LineSeparator(int $length = 40, string $character = '-') {
$break = str_repeat($character, $length);
print_r("{$break}\n");
}
}

Let’s start with a basic function called assertEquality(), which asserts that both parameters are equal to each other:

function assertEquality($a, $b) {
try {
// Assert equality.
if (assert($a == $b, "assert($a == $b) failed.")) {
Logging::Log("assert($a == $b) was successful.");
}
} catch (AssertionError $error) {
// Output expected AssertionError.
Logging::Log($error);
} catch (Error $error) {
// Output any unexpected errors.
Logging::Log($error, false);
}
}

We’ll test this out by calling it twice with two sets of arguments:

// Assert that 2 == 2.
assertEquality(2, 2);
// Assert that 1 == 2.
assertEquality(1, 2);

We’d expect that the first call would succeed, while the second will fail since 1 != 2, but our log output shows both were successful:

assert(2 == 2) was successful.
assert(1 == 2) was successful.

What gives? As it turns out, PHP 7 comes with a default php.ini configuration setting for the zend.assertions value of -1. As you can see from the description taken from the php.ini file itself, this means that assert() function calls are not even compiled:

[Assertion]
; Switch whether to compile assertions at all (to have no overhead at run-time)
; -1: Do not compile at all
; 0: Jump over assertion at run-time
; 1: Execute assertions
; Changing from or to a negative value is only possible in php.ini! (For turning assertions on and off at run-time, see assert.active, when zend.assertions = 1)
; Default Value: 1
; Development Value: 1
; Production Value: -1
; http://php.net/zend.assertions
zend.assertions = -1

We can resolve this by setting zend.assertions = 1. In addition, there’s also the assert.active setting that defaults to On, so we typically don’t need to change it, but it’s worth double-checking if your own installation is misbehaving.

Now, after running our two assertion tests again we get much different output for the second (failing) assertion call:

assert(2 == 2) was successful.

PHP Warning: assert(): assert(1 == 2) failed. failed in /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php on line 98
PHP Stack trace:
PHP 1. {main}() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:0
PHP 2. executeExamples() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:134
PHP 3. assertEquality($a = *uninitialized*, $b = *uninitialized*) /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:81
PHP 4. assert(*uninitialized*, *uninitialized*) /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:98

We’re starting to get somewhere now that our assert(1 == 2) test failed, but the PHP engine issued a Warning instead of an Error that we expected. This means that our try-catch block wasn’t able to catch the AssertionError that we were after.

This failure to throw an AssertionError is due to another php.ini setting of assert.exception. When this is set to On (or 1), it forces the PHP engine to throw an AssertionError when an assertion fails, rather than the default of issuing a warning only. Again, the official documentation description further elaborates:

  • 1: throw when the assertion fails, either by throwing the object provided as the exception or by throwing a new AssertionError object if exception wasn’t provided
  • 0: use or generate a Throwable as described above, but only generate a warning based on that object rather than throwing it (compatible with PHP 5 behaviour)

So we see that, as of PHP 7, the default behavior is to generate a Throwable if one is passed (as the second parameter), but not to actually throw it (unless assert.exception is enabled).

In our case, after setting assert.exception = On and rerunning our tests, we get the following output:

assert(2 == 2) was successful.

[EXPECTED] AssertionError: assert(1 == 2) failed. in /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php on line 112

Now we’re starting to get somewhere, as we were able to throw and catch the produced AssertionError. Yet assert() has a few more options that can be used to modify its behavior; namely the assert_options() function:

// Enable assertion (Default: true)
assert_options(ASSERT_ACTIVE, true);
// Enable warning when assertion fails (Default: true)
assert_options(ASSERT_WARNING, true);
// Enable termination if assertion is failed (Default: false)
assert_options(ASSERT_BAIL, true);

// Assert that 1 == 2.
assertEquality(1, 2);
// Assert that 2 == 2.
assertEquality(2, 2);

Here we’ve configure a few of the assert() behavioral options, most of which are available in the php.ini as well, setting them to their default values. However, we’re also setting ASSERT_BAIL to true, which will attempt to terminate the executing process if a failed assertion is encountered. Thus, if we reorder our assertEquality() function tests so our failed test comes first (as seen above), our output dramatically changes once again:

PHP Fatal error: Uncaught AssertionError: assert(1 == 2) failed. in /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:113
Stack trace:
#0 /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php(113): assert(false, 'assert(1 == 2) ...')
#1 /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php(95): assertEquality(1, 2)
#2 /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php(149): executeExamples()
#3 {main}
thrown in /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php on line 113
PHP Stack trace:
PHP 1. {main}() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:0
PHP 2. executeExamples() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:149
PHP 3. assertEquality() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:95
PHP 4. assert() /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:113

That’s quite a mess, but what’s important to note is that this produces an uncaught error. This is because ASSERT_BAIL = true forces the process to immediately terminate the moment the failed assertion is encountered, rather than completing execution. Thus, surrounding catch blocks and the like are never processed.

The last setting we’ll mess with is ASSERT_CALLBACK, which allows us to specify a callback function that will receive a handful of arguments when an assertion fails, which we can use for additional, custom processing. For our purposes we have the onAssertionFailure() function, which just accepts the handful of arguments that are passed and outputs a readable failure message:

/**
* @param $fileName Name of the executed script file.
* @param $line Code line of failed assertion.
* @param $ph Unknown placeholder.
* @param $message Assertion description.
*/
function onAssertionFailure($fileName, $line, $ph, $message) {
Logging::Log("Assertion at $fileName:$line failed with message: $message");
}

// Call a custom callback function for assertion failure.
assert_options(ASSERT_CALLBACK, 'onAssertionFailure');

// Assert that 1 == 2.
assertEquality(1, 2);

Executing our failed assertEquality(1, 2) function one last time with an ASSERT_CALLBACK function specified produces our new, modified output:

Assertion at /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php:115 failed with message: assert(1 == 2) failed.
[EXPECTED] AssertionError: assert(1 == 2) failed. in /media/sf_Airbrake.io/Exceptions/PHP/Error/AssertionError/code.php on line 115

We still generate and throw an AssertionError, but the callback function is a convenient way to handle such failures, regardless of any try/catch or assertion exception settings.

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