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PHP: Predicates

Remember the isInfant() function from the last lesson?


function isInfant($age)
    return $age < 1;


This kind of function is called a predicate. Predicate functions answer a question and always (without exception!) return either true or false.

Predicates usually have handy names in every language so that they're easily analyzed. In PHP, predicates usually begin with, but are not limited to, the prefixes is, has or can. Examples:

  • isInfant() — «is an infant?»
  • hasChildren() — «does they have children?»
  • isEmpty() — «is it empty?»
  • hasErrors() — «are there any errors?»

A function can only be considered a predicate if it returns a bool.

Let's write one more predicate function. It takes a string and checks if it's the word 'Castle':


function isCastle($type)
    return $type === 'Castle';



Write the function isMister() that takes a string and checks if it's the word 'Mister'.


isMister('Mister'); // true
isMister('Missis'); // false
The exercise doesn't pass checking. What to do? 😶

If you've reached a deadlock it's time to ask your question in the «Discussions». How ask a question correctly:

  • Be sure to attach the test output, without it it's almost impossible to figure out what went wrong, even if you show your code. It's complicated for developers to execute code in their heads, but having a mistake before their eyes most probably will be helpful.
In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

In some rare cases, the solution may be adjusted to the tests, but this can be seen immediately.

I've read the lessons but nothing is clear 🙄

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  • A predicate is an expression that answers a yes or no question using the bool data type.

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