Validate +62 Phone Number

<?php

function checkNumber($phone) {
    $verifiedNumber = NULL;

    if ( $phone ) {
        $f = substr($phone, 0, 1);
        if ( $f == "0" ) {
            $r = substr($phone, 1, strlen($phone));
            $verifiedNumber = "+62$r";
        } else if ( $f == "+" ) {
            $r = substr($phone, 1, strlen($phone) - 1);
            $cc = substr($r, 0, 2);
            if ( $cc == "62" ) {
                $verifiedNumber = $phone;
            }
        } else if ( $f == "6" ) {
            $cc = substr($phone, 0, 2);
            if ( $cc == "62" ) {
                $verifiedNumber = "+$phone";
            }
        }
    }
        
    return $verifiedNumber;
}

echo "VALID => " . checkNumber("0811111111112") . "\n";
echo "VALID => " . checkNumber("+62811111111112") . "\n";
echo "VALID => " . checkNumber("62811111111112") . "\n";
echo "INVALID => " . checkNumber("811111111112") . "\n";
echo "INVALID => " . checkNumber("63811111111112") . "\n";
echo "INVALID => " . checkNumber("+63811111111112") . "\n";