PHP Bad Practice: Variable Reuse

Anyone who has worked with PHP knows that it is extremely permissive with variables and their types. There's no need to declare variables, not even at class level, and data types cannot really be enforced. This is one of the greatest strengths of weakly typed dynamic languages, but it can be easily used the wrong way.

The problem

 Variable reuse is one such unfortunate case, which can easily lead to unmaintainable code.

Let me illustrate by this example:

$fruit = array(1 => “orange”, 2 => “banana”);
// Some code using the $item array
$fruit = $fruit[2];

As you can see, the $fruit variable starts out as an associative array and is used as such to perform various tasks. After a while, it gets reassigned and points to a string typed value inside the array.

Now what if you need some other items from your fruit array in some newly added code? You will not be able to do that, since the array is no longer available. This is especially troublesome if the functionality is added by someone else. That developer will need to figure out what is going on, modify the existing code and only afterward implement the new feature.

During our various migration projects, I have regularly encountered code that uses this annoying pattern. Such constructs are not yet explicitly handled by our type inference algorithm and so we end up with Java variables of java.lang.Object type. This is obviously not a nice thing in the generated code.

Not even larger PHP code base is free from variable reuse. From the open-source world, phpBB and WordPress come to my mind as heavy users of this construct.

The solution

Variables should be named consistently, based on the data they hold. While regarding variable types and their declarations the opinions are diverging (see static vs. dynamic typing), the meaning of the variable content is fundamental and should be reflected by its naming.

With this in mind, the above code should rather look like this:

$fruits = array(1 => “orange”, 2 => “banana”);
// Some code using the $item array
$favourite_fruit = $fruits[2];

Now isn't this much cleaner and maintainable? Had I written this, it would make me sleep better at night :-)