Find the highest prime number given an argument

highest-prime.php

<?php
class primeTest
{
	private $primeArray;

	public function __construct() {
		$this->primeArray = array();
	}
	
	private function isPrime($n) {
		if (in_array($n, $this->primeArray)) {
			return true;
		} else {
			if (sizeof($this->primeArray) > 0) {
				$top = max($this->primeArray);
			} else {
				$top = 0;
			}
			if ($n < $top) {
				return true;
			} else {
				$i = 2;
				if ($n == 2) {
					$this->primeArray[] = $n;
					return true;	
				}
				while ($i < $n) {
					if ($n % $i == 0) {
						return false;
					}
					$i++;
				}
				$this->primeArray[] = $n;
				return true;
			}
		}
	}

	public function findTopPrime($input) {
		if ($input == '') {
			return "You Must Enter A Number For This Excercise";
		} else {
			$primes = '';
			$highestPrime = 1;
			for ($i=1; $i<=$input; $i++) {
				if ($input % $i == 0) {
					if ($this->isPrime($i)) {
						if ($primes == '') {
							$primes = $i;
						} else {
							$primes .= ','. $i;
						}
						if ($i > $highestPrime) {
							$highestPrime = $i;
						}
					}
				}
			}
			$output = "Highest Prime: " . $highestPrime . "\n\n";
			$output .= "Collection of Primes: " . $primes . "\n";
			return $output;
		}
	}
}

$input = $argv[1];

$objPrime = new primeTest();
$results = $objPrime->findTopPrime($input);
echo $results;
?>