fibonacci.php

<?php 
// A simple PHP program to  
// find index of given 
// Fibonacci number. 
  
function findIndex($n) 
{ 
      
    // if Fibonacci number 
    // is less than 2, 
    // its index will be  
    // same as number 
    if ($n <= 1) 
        return $n; 
  
    $a = 0; $b = 1; $c = 1; 
    $res = 1; 
  
    // iterate until generated  
    // fibonacci number  
    // is less than given 
    // fibonacci number 
    while ($c < $n) 
    { 
        $c = $a + $b; 
          
        // res keeps track of  
        // number of generated  
        // fibonacci number 
  
        $res++; 
        $a = $b; 
        $b = $c; 
    } 
    return $res; 
}

// Driver Code 
$result = findIndex(21); 
echo($result);
?>