How to check if a given number is Fibonacci number ? | A utility function that returns true if x is perfect square ; Returns true if n is a Fibinacci Number , else false ; n is Fibinacci if one of 5 * n * n + 4 or 5 * n * n - 4 or both is a perferct square ; Driver Code
< ? php function isPerfectSquare ( $ x ) { $ s = ( int ) ( sqrt ( $ x ) ) ; return ( $ s * $ s == $ x ) ; } function isFibonacci ( $ n ) { return isPerfectSquare ( 5 * $ n * $ n + 4 ) || isPerfectSquare ( 5 * $ n * $ n - 4 ) ; } for ( $ i = 1 ; $ i <= 10 ; $ i ++ ) if ( isFibonacci ( $ i ) ) echo " $ i ▁ is ▁ a ▁ Fibonacci ▁ Number ▁ STRNEWLINE " ; else echo " $ i ▁ is ▁ a ▁ not ▁ Fibonacci ▁ Number ▁ STRNEWLINE " ; ? >