Gap in Primes https://www.codewars.com/kata/gap-in-primes/train/php

gap_in_primes.php

<?php

/*
 * @see https://www.codewars.com/kata/gap-in-primes/train/php
 */

function gap( $g, $m, $n ) 
{
    $prime = FALSE;

    for ( $i = $m; $i < $n; $i ++ ) {
        $isPrime = isprime($i);
        if ( $isPrime && $prime && $i - $prime == $g ) {
            return [ $prime, $i ];
        }
        if ( $isPrime ) {
            $prime = $i;
        }
    }

    return NULL;
}

function isprime( $n ) 
{
    for ( $i = 2; $i <= sqrt($n); $i ++ ) {
        if ( $n % $i == 0 ) {
            return FALSE;
        }
    }

    return TRUE;
}