Add problem 4 from Project Euler

Author: rubendelblanco

Maths/ProjectEuler/Problem4.php

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * Problem:
+ * A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
+ * Find the largest palindrome made from the product of two 3-digit numbers.
+ */
+
+/**
+ * @return int
+ */
+function problem4(): int
+{
+    $largest  = 0;
+
+    for ($i=100; $i<1000; $i++){
+
+        for ($j=$i; $j<1000; $j++) {
+            $product = $i*$j;
+
+            if ( strrev((string)$product) == (string)$product && $product > $largest) {
+                $largest = $product;
+            }
+        }
+    }
+
+    return $largest;
+}

tests/Maths/ProjectEulerTest.php

@@ -6,6 +6,7 @@
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem1.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem2.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem3.php';
+require_once __DIR__ . '/../../Maths/ProjectEuler/Problem4.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem5.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem6.php';
 require_once __DIR__ . '/../../Maths/ProjectEuler/Problem7.php';
@@ -29,6 +30,11 @@ public function testProblem3(): void
         $this->assertSame(6857, problem3());
     }
 
+    public function testProblem4(): void
+    {
+        $this->assertSame(906609, problem4());
+    }
+
     public function testProblem5(): void
     {
         $this->assertSame(232792560, problem5());