# PHP: Combining Operations and Functions

Logical operations are expressions. So, you can combine them with other expressions.

Let's say, for example, we want to check if a number is even. In programming, we do this:

• check the remainder of a division by 2:
• if the remainder is 0, then the number was even
• if the remainder is not 0, then the number was odd

The remainder of a division is an elementary but crucial concept in arithmetic and algebra, and even number theory and cryptography. The idea is simple; divide a number into several equal groups, and if anything remains, it's the remainder of the division.

Split some candies equally among individuals:

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

The `%` operator calculates the remainder of a division (not to be confused with division itself):

• `7 % 2``1`
• `21 % 3``0`
• `19 % 5``4`
``````<?php

7 % 2;  // 1
21 % 3; // 0
19 % 5; // 4

// Even/odd check

10 % 2 // 10 is even, because the remainder is 0
9 % 2  // 9 is odd, because the remainder is 1
``````

Write a function to check whether a number is even:

``````<?php

function isEven(\$number)
{
return \$number % 2 === 0;
}

isEven(10); // true
isEven(3);  // false
``````

In one expression, we combined the logical operator `===` (equality check) and the arithmetic operator `%`.

Arithmetic operators have higher priority than logical ones. So first the arithmetic expression `\$number % 2` is calculated, then the result is involved in a logical comparison.

We can express this in normal words like so: «calculate the remainder from dividing the number `\$number` by 2 and compare whether the remainder is zero; then return the result of the even/odd check».

Another example is to write a function to check if the first letter of a string is uppercase.

Algorithm:

1. Let's get the first character of the argument string and write it to a variable.
2. We'll compare whether the symbol is equal to its large (capital) version.
3. Let's get the result back.
``````<?php

function isFirstLetterInUpperCase(\$string)
{
\$firstLetter = \$string[0];
\$isSame = strtoupper(\$firstLetter) === \$firstLetter;
return \$isSame;
}

isFirstLetterInUpperCase('marmont'); // false
isFirstLetterInUpperCase('Robb'); // true
``````

We used the `strtoupper()` function from the standard PHP library.. It takes the string and returns a version of the string in which all the letters have been capitalized. We pass it just one character, the first character of the string.

Try and explain what's happening in simple English, similar to how we decoded the process in the `isEven()` example at the beginning of the lesson.

Do you remember how we extract characters from a string with square brackets?

``````<?php

\$firstName = 'Alexander';

\$firstName[0]; // A
``````

## Instructions

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
``````
The exercise doesn't pass checking. What to do? 😶

• 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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