We already know how to write functions that check single conditions. In this lesson, we'll learn how to build compound conditions.

Suppose that on signup, a website requires a password to be longer than eight characters and to contain at least one capital letter. Let's try writing two separate logical expressions and connecting them with the special «AND» operator:

A password longer than 8 characters AND a password that contains at least one capital letter

Here's a function that takes a password and says whether it meets the conditions (true) or not (false):

<?php

function hasCapitalLetter(string $text): bool
{
    // Checks for at least one capital letter in the string
}

function isCorrectPassword(string $password): bool
{
    $length = strlen($password);

    return $length > 8 && hasCapitalLetter($password);
}

var_dump(isCorrectPassword('Qwerty')); // => bool(false)
var_dump(isCorrectPassword('Qwerty1234')); // => bool(true)
var_dump(isCorrectPassword('qwerty1234')); // => bool(false)

The && operator means «AND». In mathematical logic, this is called conjunction. The whole expression is considered true if every operand is true, that is, each of the compound expressions. In other words, && means «both this and that». This operator's priority is lower than that of comparison operators. That's why the expression hasCapitalLetter($password) && $length > 8 also works correctly without parentheses.

Besides &&, the || operator, which means «OR» (disjunction), is often used. It means «either this, or that, or both». The expression $a || $b is considered true if at least one of the operands is true or when both operands are true. Otherwise, the expression is false.

<?php

function hasDiscount(int $age, bool $isStudent): bool
{
    return $age < 18 || $isStudent;
}

var_dump(hasDiscount(15, false)); // => bool(true)  (younger than 18)
var_dump(hasDiscount(25, true)); // => bool(true)  (student)
var_dump(hasDiscount(15, true)); // => bool(true)  (both conditions)
var_dump(hasDiscount(25, false)); // => bool(false)

Operators can be combined in any number and in any sequence. If && and || appear together in the code, it's better to set the priority with parentheses. Below is an example of an extended function that determines whether a password is correct:

<?php

function hasCapitalLetter(string $text): bool
{
    // Checks for at least one capital letter in the string
}

function hasSpecialChars(string $text): bool
{
    // Checks for special characters in the string
}

function isStrongPassword(string $password): bool
{
    $length = strlen($password);

    // The parentheses set the priority. It's clear what relates to what.
    return $length > 8 && hasCapitalLetter($password) && hasSpecialChars($password);
}

Now let's imagine that we want to buy an apartment that meets these conditions: an area of 100 square meters or more on any street OR an area of 80 square meters or more, but on the central Main Street.

Let's write a function that checks the apartment. It takes two arguments: the area (a number) and the street name (a string):

<?php

function isGoodApartment(int $area, string $street): bool
{
    return $area >= 100 || $area >= 80 && $street === 'Main Street';
}

var_dump(isGoodApartment(91, 'Queens Street')); // => bool(false)
var_dump(isGoodApartment(78, 'Queens Street')); // => bool(false)
var_dump(isGoodApartment(70, 'Main Street')); // => bool(false)

var_dump(isGoodApartment(120, 'Queens Street')); // => bool(true)
var_dump(isGoodApartment(120, 'Main Street')); // => bool(true)
var_dump(isGoodApartment(80, 'Main Street')); // => bool(true)

The area of mathematics that studies logical operators is called Boolean algebra. Truth tables show what the result will be when each operator is applied.

AND &&

OR ||

Implement the isLeapYear() function, which determines whether the year is leap year or not. A leap year is a multiple of 400 (i.e. divisible without a remainder), or is both a multiple of 4 and not a multiple of 100. As you can see, the definition already contains all the required logic, all we need to do is to put it into code:

<?php

isLeapYear(2018); // false
isLeapYear(2017); // false
isLeapYear(2016); // true

You can check if a number is a factor of another number like so:

<?php

// % - returns the remainder of the left operand divided by the right operand
// Checking that the number is a multiple of 10
$number % 10 === 0
// Checking that the number is not a multiple of 10
$number % 10 !== 0