PHP Question: Defining Constants

Question

What does the following code do?

define("MY_CONSTANT",array(1,2,3,4,5));

Answer

If this code is run it will produce the following error:

This is because only scalar values can be assigned to constants. A scalar value is any integer, float, string or boolean value and does not include arrays, objects or resources. Trying to set any non-scalar value as a constant will produce this error.

If we went on to var_dump() the constant defined above we would get back a string containing "MY_CONSTANT". This is because if you try to retrive any constant value that hasn't been set PHP will try to evaluate it as a string. What this means is that if you try to do a simple boolean test for the constant (see the example below) it will evaluate to true.

1
2
3

if(MY_CONSTANT){  echo'constant set, or not...';}

We can test for the existence of the constant in a couple of ways. We already know that any constant that isn't defined is cast as a string so we can test for this by doing a type comparison between the constant and a string of the constant name. If the constant is defined then this will equal false.

However, when we do this we get the following notice about using an undefined constant and that PHP has cast it as a string.

The built in PHP function defined() can be used to test for the existence of a constant value. This accepts a string of the constant name and will return a boolean value that shows if the constant has been set or not. This also avoids the need to try out the constant itself and get a notice if it isn't defined.

There are a couple of ways to get around the restriction of only allowing scalar values to be used in constants. The first way is to use serialize() to convert the array into a string before setting it as a constant.

1
2
3

define("MY_CONSTANT",serialize(array(1,2,3,4,5)));var_dump(defined('MY_CONSTANT'));// prints truwprint_r(unserialize(MY_CONSTANT));

We could also use json_encode() here, which will have the same effect. This probably isn't the best way to set constant values, although it is perfectly allowed in PHP. The other way to get a constant with an array is to use a static variable in a class. This is effectively a constant as it won't (at least it shouldn't) be changed during the execution of the script.

1
2
3
4
5

class Constants {     public static $myConstant=array(1,2,3,4,5);} 
print_r(Constants::$myConstant);

This will print out the array in full.

Note that you can't set arrays to be constants in classes as you will get a fatal error instead of a warning. For example, the following script.

1
2
3
4
5

class Constants {     const MY_CONSTANT =array(1,2,3,4,5);} 
print_r(Constants::MY_CONSTANT);

Will produce the following error when run.