How to check if a given number is Fibonacci number?

Question

i am trying to check my numbers is fibonacii or not ?

isFibonacci(13);
function isFibonacci( $testedNumber, $a = 1, $b = 1 )
{
    if( $testedNumber == 0 || $testedNumber == 1 )
        return true;//returning true for 0 and 1 right away.
    $nextFib = $a + $b;//getting the next number in the sequence
    if( $nextFib > $testedNumber )
        return false;//if we have passed the tested number, it's not in the sequence
    else if( $nextFib == $testedNumber )
        return true;//if we have a perfect match, the tested number is in the sequence
    else
        isFibonacci( $testedNumber, $b, $nextFib );//otherwise, get the next fibonacci number and repeat.
} 

Answer 1

<?php

function getFibonicciIndex($number)
{
    $log_base = (1+sqrt(5))/2;
    $index = log(($number*sqrt(5)-(1/2)), $log_base);
    return floor($index)+1;
}
function getFibonicciNumber($term)
{
    $a = (1+sqrt(5))/2;
    $b = (1-sqrt(5))/2;
    $fibonicci_number = (pow($a, $term)-pow($b, $term))/sqrt(5);
    return $fibonicci_number;
}
$number = 14;
$index = getFibonicciIndex($number);
$index_value = getFibonicciNumber($index);
echo ($number == $index_value) ? "yes" : "no";

//This logic implements the best rule to find the Fibonacci series and their index. First we suppose the given no. is a part of Fibonacci series and try to get index .. and then for that given index we calculate the Fibonacci number and equate it to verify