Code Logo

Palindrome Checker

Published at24 Jul 2026
Created by
PHP String Handling Medium 0 views
Like0

Write a PHP function that checks if a string is a palindrome — it reads the same forwards and backwards. Ignore case and non-alphanumeric characters.

Convert the string to lowercase with strtolower(). Remove all non-alphanumeric characters using preg_replace('/[^a-z0-9]/', '', $clean). The cleaned string contains only letters and digits in lowercase. Compare this cleaned string with its reverse using strrev().

An alternative two-pointer approach: set $left = 0 and $right = strlen($s) - 1. While $left < $right, compare characters at both positions. If they differ, return false. If they match, move $left up by 1 and $right down by 1.

Edge cases include empty strings (true), single characters (true), strings with only non-alphanumeric characters (true after cleaning), and mixed case like 'RaceCar'.

Time complexity is O(n). Space complexity is O(n) for the cleaned string copy.

The two-pointer approach uses O(1) extra space and avoids creating intermediate strings, making it memory-efficient for long strings. The ctype_alnum() function checks if a character is alphanumeric. The two-pointer approach with ctype_alnum() and strtolower() handles all ASCII palindromes correctly without relying on regex functions that may have platform-dependent behavior.

Example Input & Output

Example 1
Input
"RaceCar"
Output
true
Example 2
Input
"Hello"
Output
false
Example 3
Input
"12321"
Output
true
Example 4
Input
"A man, a plan, a canal: Panama"
Output
true
Example 5
Input
""
Output
true

Algorithm Flow

Recommendation Algorithm Flow for Palindrome Checker
Recommendation Algorithm Flow for Palindrome Checker

Solution Approach

Write a PHP function that checks if a string is a palindrome — it reads the same forwards and backwards. Ignore case and non-alphanumeric characters.

Convert the string to lowercase with strtolower(). Remove all non-alphanumeric characters using preg_replace('/[^a-z0-9]/', '', $clean). The cleaned string contains only letters and digits in lowercase. Compare this cleaned string with its reverse using strrev().

An alternative two-pointer approach: set $left = 0 and $right = strlen($s) - 1. While $left < $right, compare characters at both positions. If they differ, return false. If they match, move $left up by 1 and $right down by 1.

Edge cases include empty strings (true), single characters (true), strings with only non-alphanumeric characters (true after cleaning), and mixed case like 'RaceCar'.

Time complexity is O(n). Space complexity is O(n) for the cleaned string copy.

Best Answers

php - Approach 1
<?php
function isPalindrome($text) {
    $left = 0;
    $right = strlen($text) - 1;
    while ($left < $right) {
        while ($left < $right && !ctype_alnum($text[$left])) $left++;
        while ($left < $right && !ctype_alnum($text[$right])) $right--;
        if (strtolower($text[$left]) !== strtolower($text[$right])) return false;
        $left++;
        $right--;
    }
    return true;
}