Palindrome program in PHP

Got it 👍 You want a Palindrome program in PHP but explained with DSA (Data Structures & Algorithms) concept.
I’ll give you:

1. Definition
2. Algorithm (step by step)
3. Example
4. PHP Code Implementation

🔹 1. Definition

A palindrome is a string/number that reads the same forwards and backwards.
Examples:

• "madam", "121", "racecar" ✅ Palindrome
• "hello", "1234" ❌ Not Palindrome

🔹 2. Algorithm (DSA Concept)

We’ll use two-pointer technique (common DSA approach):

1. Take input string str.
2. Initialize two pointers:
   • left = 0 (start of string)
   • right = strlen(str) - 1 (end of string)
3. While left < right:
   • Compare str[left] and str[right]
   • If not equal → Not a palindrome → return false
   • Else move pointers: left++, right--
4. If loop ends → It’s a palindrome

⏱ Time Complexity: O(n)
📦 Space Complexity: O(1)

🔹 3. Example

Input: "madam"

• Compare m (left) with m (right) → ✅
• Compare a with a → ✅
• Compare d with d → ✅
  → Palindrome ✅

Input: "hello"

• Compare h with o → ❌ Not palindrome

🔹 4. PHP Implementation

<?php
function isPalindrome($str) {
    $left = 0;
    $right = strlen($str) - 1;

    while ($left < $right) {
        if ($str[$left] !== $str[$right]) {
            return false; // not palindrome
        }
        $left++;
        $right--;
    }
    return true; // palindrome
}

// Test cases
$words = ["madam", "121", "hello", "racecar"];

foreach ($words as $word) {
    if (isPalindrome($word)) {
        echo "$word is a Palindrome ✅<br>";
    } else {
        echo "$word is NOT a Palindrome ❌<br>";
    }
}
?>

now solve the Palindrome problem in PHP using Stack (DSA approach).

🔹 1. Why Stack?

• Stack works on LIFO (Last In First Out).
• If we push all characters of a string into a stack and then pop them, we get the reverse of the string.
• If the reversed string == original string → It’s a palindrome.

🔹 2. Algorithm (Stack-based)

1. Take input string str.
2. Initialize an empty stack (array in PHP).
3. Push every character of the string into the stack.
4. Pop all characters from stack → This gives reversed string.
5. Compare reversed string with original:
   • If equal → Palindrome ✅
   • Else → Not Palindrome ❌

⏱ Time Complexity: O(n)
📦 Space Complexity: O(n) (since we store reversed string in stack)

🔹 3. PHP Implementation (Stack Approach)

<?php
function isPalindromeStack($str) {
    $stack = [];

    // Push all characters into stack
    for ($i = 0; $i < strlen($str); $i++) {
        array_push($stack, $str[$i]);
    }

    // Pop characters to form reversed string
    $reversed = "";
    while (!empty($stack)) {
        $reversed .= array_pop($stack);
    }

    // Compare original with reversed
    return $str === $reversed;
}

// Test cases
$words = ["madam", "121", "hello", "racecar"];

foreach ($words as $word) {
    if (isPalindromeStack($word)) {
        echo "$word is a Palindrome ✅ (Stack method)<br>";
    } else {
        echo "$word is NOT a Palindrome ❌ (Stack method)<br>";
    }
}
?>

🔹 4. Comparison of Approaches

👉 The two-pointer method is more efficient.
👉 The stack method is more intuitive for learning DSA concepts.