Aggregation applies not only to numbers but also to strings. These are tasks in which a string is formed dynamically — that is, you don't know in advance what size it will be or what it will contain.

Imagine a function that can multiply a string, that is, repeat it a specified number of times:

<?php

repeat('hexlet', 3); // 'hexlethexlethexlet'

The principle behind this function is quite simple — in a loop, the string is "built up" the specified number of times:

<?php

function repeat(string $text, int $times): string
{
    // The neutral element for strings is an empty string
    $result = '';
    $i = 1;

    while ($i <= $times) {
        // Each time we add the string to the result
        $result = "{$result}{$text}";
        $i = $i + 1;
    }

    return $result;
}

Let's break down the execution of this code step by step:

<?php

// To call repeat('hexlet', 3);
$result = '';
$result = "{$result}hexlet"; // hexlet
$result = "{$result}hexlet"; // hexlethexlet
$result = "{$result}hexlet"; // hexlethexlethexlet

Visually, the process of building up the string looks like this:

repeat('hexlet', 3):

i=1: result = ''             . 'hexlet' = 'hexlet'
i=2: result = 'hexlet'       . 'hexlet' = 'hexlethexlet'
i=3: result = 'hexlethexlet' . 'hexlet' = 'hexlethexlethexlet'
                                            └── result

Neutral element

For the building up to work, you need a starting value. For strings, this value is the empty string ''.

It is called the neutral element because it changes nothing during concatenation:

<?php

print_r('' . 'abc');  // => abc
print_r('abc' . '');  // => abc

That is why the empty string is always used as the initial value when aggregating strings.

Implement the sanitizePhoneNumber() function, which takes a phone number from a form and returns a string without spaces, parentheses, or hyphens.

Users enter numbers in different ways, but before saving they are often normalized to a single format. Go through the original string character by character and assemble a new number from useful characters only.

<?php

sanitizePhoneNumber('+7 (999) 123-45-67'); // '+79991234567'
sanitizePhoneNumber('8 800 555 35 35');    // '88005553535'
sanitizePhoneNumber('(123) 456-7890');     // '1234567890'

Hints

• The final result is a string.
• Use an empty string as the initial value.
• The str_contains() function will help you check whether a character is in the set ' ()-'.