Write a function which, taking in a positive integer n as input, returns an array of all primes lower than n.

get_primes.php

<?php
/*
Sample expected output:
getPrimes(5);   ⇒ array(2, 3)
getPrimes(10);  ⇒ array(2, 3, 5, 7)
*/

function getPrimes($n)
{
	if ($n <= 0) throw new InvalidArgumentException("positive integer required");
	if ($n < 2) return array();
 
	// 2 is the only "special" prime we care about
	$primes = array(2);
 
	// for efficiency, we iterate over odd numbers only
	for ($candidate=3; $candidate<$n; $candidate+=2)
	{
		// we use the collection of previously found primes to limit number of iterations
 
		// there's no need to go beyond the square root when looking for factors
		$upper_bound = ceil(sqrt($candidate));
 
		// local variable to avoid lookup at each loop iteration
		$len = count($primes);
 
		// for loop starts at index 1 because we have already excluded even numbers, so we don’t need to test modulo 2
		for ($i=1; $i<$len; $i++)
		{
			$cur_prime = $primes[$i];
			if ($candidate % $cur_prime === 0) continue 2;
			if ($cur_prime >= $upper_bound) break;
		}
 
		// if we reach here, we've found a new prime! yeah!!
		$primes[] = $candidate;
	}
 
	return $primes;
}
 
echo implode(',', getPrimes(200));