Create a boolean array " prime [ 0 . . n ] " and initialize all entries it as true . A value in prime [ i ] will finally be false if i is Not a prime , else true .
$ prime = array_fill ( 0 , $ N , true ) ; function SieveOfEratosthenes ( ) { global $ N , $ prime ; $ prime [ 1 ] = false ; for ( $ p = 2 ; $ p < ( int ) ( sqrt ( $ N ) ) ; $ p ++ ) {