Driver Code Check whether 2 ^ p - 1 is prime or not .
$ p = 7 ; $ checkNumber = pow ( 2 , $ p ) - 1 ; if ( isPrime ( $ p ) ) echo $ checkNumber , " ▁ is ▁ Prime . " ; else echo $ checkNumber , " ▁ is ▁ not ▁ Prime . " ; ? >