Check if a String is a Palindrome (PHP)

Problem

Write a function isPalindrome(string $s): bool that returns true for inputs like:

"racecar" → true
"A man, a plan, a canal: Panama" → true
"hello" → false

Solution

php

<?php
function isPalindrome(string $s): bool {
    // Keep only letters and digits, lowercase everything
    $clean = strtolower(preg_replace('/[^a-z0-9]/i', '', $s));
    return $clean === strrev($clean);
}

var_dump(isPalindrome("racecar"));                          // true
var_dump(isPalindrome("A man, a plan, a canal: Panama"));   // true
var_dump(isPalindrome("hello"));                            // false

Two-pointer alternative (O(1) memory, no string copy)

php

function isPalindromeTwoPointer(string $s): bool {
    $s = strtolower($s);
    $l = 0; $r = strlen($s) - 1;
    while ($l < $r) {
        while ($l < $r && !ctype_alnum($s[$l])) $l++;
        while ($l < $r && !ctype_alnum($s[$r])) $r--;
        if ($s[$l] !== $s[$r]) return false;
        $l++; $r--;
    }
    return true;
}

What interviewers look for

Edge cases — empty string is a palindrome; single char is a palindrome.
Did you handle Unicode? strrev reverses bytes, not characters. For Unicode use mb_str_split + array_reverse + implode.
Time/space complexity — both versions are O(n) time, but the two-pointer version uses O(1) extra space.

Unicode-safe version

php

function isPalindromeUnicode(string $s): bool {
    $chars = mb_str_split(mb_strtolower($s));
    $chars = array_filter($chars, fn($c) => preg_match('/[\p{L}\p{N}]/u', $c));
    $clean = implode('', $chars);
    return $clean === implode('', array_reverse(mb_str_split($clean)));
}

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