This condition is checked so that we can skip middle five numbers in the below loop
if ( $ n % 2 == 0 $ n % 3 == 0 ) return false ; for ( $ i = 5 ; $ i * $ i <= $ n ; $ i = ( $ i + 6 ) ) if ( $ n % $ i == 0 || $ n % ( $ i + 2 ) == 0 ) return false ; return true ; }