Create a boolean array " prime [ 0 . . high ] " 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 [ $ high + 1 ] = array ( ) ; $ twin = false ; $ prime = array_fill ( 0 , ( $ high + 1 ) , true ) ; $ prime [ 0 ] = $ prime [ 1 ] = false ;