PHP INTERVIEW · Episode 06

Print the Fibonacci series in PHP

The classic loop-logic interview question

ep6_fibonacci_series.php

function fibonacci($number){
  $numberOne = 0;
  $numberTwo = 1;
  $numbersArray = [$numberOne,$numberTwo];

  for($i=0;$i<$number;$i++){
      $numberThree = $numberOne + $numberTwo;
      $numbersArray[] = $numberThree;
      $numberOne = $numberTwo;
      $numberTwo = $numberThree;

  }
  return $numbersArray;
}


$numberOfDigits = 20;
 $output = fibonacci($numberOfDigits);

Output

Output for this tutorial goes here.

01123581321345589144233377610987159725844181676510946

This is the actual output....

The Question

“Write a program to print the Fibonacci series” is one of the most common logic questions in a junior interview. It checks whether you can turn a simple rule into a working loop.

The Rule

The series starts with 0 and 1, and every number after that is the sum of the previous two: 0, 1, 1, 2, 3, 5, 8, 13…

The Logic

Keep two variables — $a (current) and $b (next). On each loop: save $a, then slide the window forward by setting $a = $b and $b = $a + $b. Repeat n times. This runs in O(n) time with no extra memory beyond the result.

Example Output

The first 10 terms:

0112358132134

Each number is the sum of the two before it (0+1=1, 1+1=2, 1+2=3…).

Key takeaway: track the last two numbers and keep adding them. The loop approach is simple, fast, and won’t blow up for large n.

Interview follow-up: they’ll often ask for a recursive version too — fib(n) = fib(n-1) + fib(n-2). It’s elegant but slow (O(2^n)) because it recomputes the same values. The fix: memoization (cache results) or the iterative loop above.

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