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PHP best practice: Function Parameters

I’ve been developing web applications for some years now, and while I make no claims to being the world greatest developer, I do figure, that I do have some solid experience which may help others – or at least encourage thoughts and discussion. This is the first in a series of posts, and while it may be from a PHP developers point of view, it may applicable to other programming languages, and maybe even beyond web applications. Here are my four tips on function parameters.

Always have a default value on all parameters

Functions parameters are often used as input to SQL queries, calling webservices or computations. Null or non-existing values often tend to break these things and throws horrible messages to the end-user.

While the parts of the program using your function always should provide proper values, laziness, oversights or unexpected scenarios, may prove it not to be the case. If at all possible, always try to provide default values on all parameters to a function – and if you really can’t make sure it’s handled gracefully.

Choose reasonable defaults

When providing default values, don’t choose extremes. If you’re browsing in a list of usernames, don’t use a (pure) wildcard as default value – use an “A” to list all users starting with the letter A. If you’re function allows a limit on the number of rows returned form a database query (say for paging purposes), set the default to 10, 20 or some other low number, don’t go for worst case scenarios like 9999, or 999999.

If the developer using the function needs plenty of rows, it’s easy to pass a specific value, and if the developer forgets to specify the number of rows, expected, they get a reasonable result, which may help them to ask for more (if actually needed).

Always sanitize input

Even though a given function naturally only will be used with valid input and so on, every function should take steps to secure them selves.
One of the most basic steps is to make sure all input is sanitized. Protecting your function from making SQL injection threats or other security issues, is not the responsibility of the places utilizing the function, but the responsibility of the function it self, and thus it should take steps to make sure it doesn’t introduce security issues.

As basic validation of simple input parameters, look at the ctype functions in PHP. If you can always try to validate against a whitelist (which characters are allowed) instead of blacklists (these characters are not allowed) as missing things which may introduce issues in a black list is harder, than allowing what you expect, to pass through.

Accept an array of key value pairs

If a simple function accepts a single or two parameters, they may be passed as regular parameters, but once the list of possible parameters starts to grow, changing it a single parameter – which is an array with key/value-pairs, seems to be a much more solid long-term solution on keeping the interface developer friendly. Sure you can have endless lists of 10-15 parameters, but if you have default values on the first 14 values and just need to change the last, the code ought to be much more clean by being able to pass an array with the single key/value-pair needed to be changed from the default value.

That was the first four tips on function parameters. More on PHP and Security.