Normalize phone number inputs from user to E.164 format.

e164_format.php

<?php

/**
 * @param string|null $number
 * @return string|null
 */
function normalize_to_e164(?string $number): ?string
{
    if (empty($number)) {
        return null;
    }

    $number = preg_replace('/\D/', '', $number);
    if (strpos($number, '00') === 0) {
        $number = substr_replace($number, '+', 0, 2);
    }

    $number = ltrim($number, '0');
    if ($number[0] !== '+') {
        $number = '+91'.$number;
    }

    if (strlen($number) === 15 && strpos($number, '+9191') === 0) {
        $number = substr_replace($number, '', 2, 2);
    }

    return $number;
}