(This is an update on a blog post I wrote last year about parameter type checking)
PHP is dynamically typed
PHP is a dynamically typed language. What this means is that it allows you to do things like :
$a = 5;
$a = 'test';
$a = false;
The reason this works, is because PHP enforces type rules during execution, not at compile-time.
In many other languages this is impossible, since you need to define a type for the variable at compile-time. Languages such as Java, C, C++, C# and VB.Net are good examples of statically typed languages.
Problems with dynamic typing
Although dynamic typing is considered to be one of PHP’s strong suites, it does pose some problems. Let me illustrate with an example :
Suppose we have a piece of code that processes the amount of money each employee must be paid. Employees can file expense notes that are paid back in cash or when their monthly wages are paid. Our code will make the calculation for the pay check.
The data for our 4 employees is located in a CSV-file, made available from an external source :
employeeId, firstname, lastname, wage, expenses, processexpenses
1, Claire, Clarckson, 2000, 212, 0
2, Tom, Whitney, 1910, 111, 0
4, Jules, verne, 1932, 98, 1
5, Gregory, Jameson, 2131, 241, 0
If the last field is true, the expenses must be added to wage amount on the paycheck. So our code might look like this (don’t pay attention to code quality, it’s an example of ‘the average piece of code you will find’) :
class Wages
{
/**
* Process the wages
* @return boolean
*/
public function processWages()
{
$handle = fopen('some-file.csv', 'r');
while (($data = fgetcsv($handle, ',')) !== false) {
if (is_numeric($data[0])) {
$result = $this->processLine($data[2], $data[3], $data[4]);
$this->sendPaycheck($data[0], $result);
}
}
return true;
}
/**
* Calculate wages based on processExpenses parameter
*
* @param float $wage
* @param float $expenses
* @param boolean $processExpenses
* @return float
*/
private function processLine($wage, $expenses, $processExpenses)
{
if ($processExpenses) {
return $wage + $expenses;
} else {
return $wage;
}
}
/**
* Pay the employee
*
* @param int $id
* @param float $amount
*/
private function sendPaycheck($id, $amount)
{
echo 'Paycheck for id ' . $id . ' for the amount of : ' . $amount ."\n";
}
}
Everything works fine, until the external source decides (probably unknowingly) to modify the data format to :
employeeId, firstname, lastname, wage, expenses, processexpenses
1, Claire, Clarckson, 2000, 212, false
2, Tom, Whitney, 1910, 111, false
4, Jules, verne, 1932, 98, true
5, Gregory, Jameson, 2131, 241, false
When we run the code, everyone will be paid their expenses, even those that have ‘false’ in the last field. The reason ? The last field of each line might look like a boolean, but is in fact a string. The “false” is read as a string and is boolean true.
You might say that we didn’t follow best coding practices in our :
if ($processExpenses) {
which should have been
if ($processExpenses === true) {
but that would only have reversed the effect : nobody would have been paid.
Similar non-boolean situations cause the same problem. There’s a huge list of problems that might be caused by passing incorrect types to a function.
Granted, we should have put a type check in place, but as I said this was the average type of code you will find. And it’s exactly this type of code I wanted to use for this demo.
So what’s the solution ?
Since we don’t want to give up on our dynamic typing, we need a way to verify that parameters being passed to a function/method are of the type that we intend them to be. That way, anyone who wishes to use our function/method will be forced to pass the right parameter.
One solution would be to use type safe objects like the ones described by Sebastian Bergman (author of PHPUnit) in his Type-Safe Objects in PHP presentation. However, this is unusable for existing projects as it requires a massive rewrite. Furthermore, as Sebastian indicates, it poses a lot of new problems. And finally, it slows things down quite a bit, since it uses reflection to verify types during execution…
Another solution would be to have type hinting for all types, including scalar types, in PHP. Although proposed and agreed to by many, the current concensus (as of today at least) is to not include it PHP 5.4. It might end up in a branch for future use or might end up as a PHP extension down the line, but for now it seems to be off the table for PHP 5.4.
So should or shouldn’t we check the type of a parameter before using it ?
Another big dillema : should you check each parameter’s type in each single function at runtime using is_int, is_bool, etc. ? Some would say it’s the safest way and the only way to be absolutely sure.
I believe there’s a different and better approach : if you can integrate type checking in your unit testing and have a high code coverage percentage, there’s no need to explicitely check the type during runtime, except when handling external data.
So how do we make sure types are checked ?
What system is better suited for the job than the most popular testing PHP framework, PHPUnit ?
Since PHPUnit run can be repeated over and over again and introducing additional checks will not cause performance issues on the actual production environment, this is a good place to add these checks.
Upon execution of each PHPUnit Testcase, this patch will verify parameters for each of the called functions/methods. You can define the depth of calls using a PHPUnit command line parameter (the default is 2, which means functions called from the testcase itself and functions called from those functions).
How does the system know what types are expected if we don’t have type hints ?
The system presumes that if you’re using PHPUnit, you clearly know proper development methods. This also means you’ll be using docblocks to comment your functions.
So, since there are no type hints to rely on, it will instead rely on the types you specify in the docblock
It will analyze the docblock of each function/method and compare each parameter type with the expected parameter type. If it finds an inconsistency, it will produce a PHPUnit warning.
So does it support…
– Classes : yes
– Interfaces / Abstract classes / parent classes : yes, in fact if you specify an interface or abstract/parent class in the docblock and pass a class implementing/extending them, it will detect it as a valid type
– Multiple types definitions in the docblock : yes – separate them by a pipe (|)
– Return value types : yes – needs Xdebug patch, see below
Is it perfect ?
Nothing is… there’s a few problems at this point :
– If you want to analyze return values, you’ll need a patch for Xdebug I wrote last week. You can download that patch here : XDebug bug #416 patch
– It still needs a bit of tuning… it’s a work in progress !
The MySQL problem : something most people don’t know
Data from any external source might cause problems. MySQL is the best example : any data being returned from MySQL using the non-binary protocol is a string, even if the column is defined as int, decimal, bool (tinyint), …
MySQL’s protocol returns all data as a string and the PHP mysql and mysqli extensions don’t convert it into the expected datatype. The result is that any data from MySQL will be passed as a string, which can cause havoc when doing type checks. The only exception is when using mysqlnd and prepared statements (see the second example of Scalar type hints in PHP trunk).
There are 3 solutions to this problem :
- Cast everything : not really fun, since you’ll need to change all your code. It might also be bad for performance, although it does set the types right ofcourse. Might be the best option for new code.
- Use Propel or some other database layer, which does the casting for you… same performance problem ofcourse.
- Wait until someone makes changes to the way MySQL and PHP communicate…
In the meantime, if you want to use the type checking, but you have some problems with MySQL, you can use a docblock tag to disable type checking for some functions : @phpunit-no-type-check
How to run it
After applying the patch to PHPUnit (and Xdebug if you want return type checking), run it like this :
php [path to phpunit.php] --check-param-types [TestCase.php|TestSuite.php]
Optional parameter :
–check-param-type-depth= sets the depth to which it needs to check parameter types. Your test is depth 0, any called function within your test is 1, etc. – default is 2 although 3 might be handy too
The output
This is the kind of output you can expect :
2) ATest::testMultiply
Invalid type calling A->multiply : parameter 2 ($factor2) should be of type int but got string(1) instead in C:\development\Test\ATest.php:42
Note that if you use the ‘–check-param-type-depth’ parameter and set it to a high number, you might see errors about libraries you use. Ofcourse, that might be the right moment to notify the library author (or contribute a fix yourself !)
Advantages
- No need to use is_int, is_bool, etc. in a function that was type-checked
- Consistent use of types
- Developers will learn to focus not just on the content of a variable, but on the type as well. In time, it will become second nature to use the correct types from the start. Code hinting in most IDEs should in fact already help out with this, but now there’s a way to verify this too.
- Verification of each function/method call, not just in terms of functionality (PHPUnit’s job), but also in terms of data types
- Forces developers to keep the docblock up-to-date (!)
Using type checking basically brings the best of the dynamically and statically typed worlds together : you still have the flexibility of dynamic typing, but assurance that functions are called with the parameter types they were designed to be called with (as well as return the correct types). It’s the perfect middle-of-the-road approach for teams with a mix of ‘strict’ and ‘not-so-strict’ developers.
Where to get it
The PHPUnit modification can be downloaded through a Github-fork : http://github.com/wimg/phpunit
Possible extension
There’s plenty of things that could be added.
One of them is “non-strict” option, which ignores type conversion between types as listed on http://wiki.php.net/rfc/typecheckingstrictandweak (option 2 section)
Feedback
As always, feedback is much appreciated, as a comment in this blog or via e-mail.
