Logical operations are expressions, so they can be combined with other expressions. For example, let's say we want to check whether a number is even, that is, whether it's a multiple of two. In programming, to do this we check the remainder of the division by two. If the remainder is 0, the number is even. If the remainder is not 0, the number is odd.

The remainder of a division is a simple but important concept in arithmetic, algebra, number theory, and cryptography. You divide a number into several equal groups, and if anything is left at the end, that is the remainder of the division.

Splitting candies equally among people:

• 7 candies, 2 people: 2 x 3 + remainder 1 (7 is not a multiple of 2)
• 21 candies, 3 people: 3 x 7 + remainder 0 (21 is a multiple of 3)
• 19 candies, 5 people: 5 x 3 + remainder 4 (19 is not a multiple of 5)

The % operator calculates the remainder of a division:

• 7 % 2 → 1
• 21 % 3 → 0
• 19 % 5 → 4

Let's combine the logical operator "equality check" === and the arithmetic operator % in a single expression and write a function that checks whether a number is even:

<?php

function isEven(int $number): bool
{
    return ($number % 2) === 0;
}

var_dump(isEven(10)); // => bool(true)
var_dump(isEven(3)); // => bool(false)

Arithmetic operations have a higher priority than logical ones. That means the arithmetic expression $number % 2 is calculated first, and then the result is compared with zero, and the result of the equality check is returned.

Now let's write a function that takes a string and checks whether this string starts with the Latin letter a.

Algorithm:

1. Get the first character of the argument string and store it in a variable
2. Compare whether the character is equal to the Latin letter a
3. Return the result

<?php

function isFirstLetterAnA(string $text): bool
{
    $firstLetter = $text[0];

    return $firstLetter === 'a';
}

var_dump(isFirstLetterAnA('orange')); // => bool(false)
var_dump(isFirstLetterAnA('apple')); // => bool(true)

To make it clear what's happening here, try to say out loud what's going on, just like we decoded the process in the isEven() example.

Now you know that comparison operations are used in programming on a par with arithmetic ones. But remember that equality is denoted by ===. That way you won't confuse this operation with assigning a value to a variable.

Implement the isInternationalPhone(), function, which checks the format of the passed phone number. If the number starts with +, then it's in the international format.

<?php

isInternationalPhone('89602223423'); // false
isInternationalPhone('+79602223423'); // true