How to Pass Array Back and Forth from Functions in PHP

I’ve been working on a project where I need to run a series of functions in PHP on an ordered array of values. I also have a separate series of if-else statements that determine the order in which these functions are called. Every component seems to be working perfectly when run in isolation, but I couldn’t get some of the functions to pass the array back for further use. No matter what I tried, when I called print_r to output the array, nothing appeared as expected.

I’m going to share my journey through this issue, show you my code, explain what was going wrong, and finally, share what I learned.

The Problem

Here’s a simplified version of the code that was giving me trouble:

public function add(&$b)
{
  global $a;

  $b[] = current($a);
  echo current($b);
  next($a);
  next($b);

  return $b;
}

public function addlots()
{
  $i = 0;
  $how_many = 5;
  while ($i < $how_many):
    parent::add($b);
    $i++;
  endwhile;

  if ($value == "start")
  {
    $run = new funcclass();
    $run->init();
    $run->addlots();
    print_r($b);  
    unset ($value);
  } else {
    // Other logic...
  }
}

I expected that after several calls to add(), my array $b would accumulate the values from the global array $a and be available for printing. However, when I ran the code, print_r($b) didn’t output the results I anticipated.

What I Learned

After debugging for hours, I discovered a few key points that were causing the issue:

1. Passing by Reference vs. Value:
   I was correctly passing $b by reference in the add() function with &$b, but the problem was that I wasn’t properly managing the variable $b outside the function. Since PHP functions operate on a copy of a variable by default, using references requires that the variable is already defined and maintained in the calling scope.
2. Global Variables:
   I was using global $a inside the function, which isn’t always the best practice as it can lead to unexpected behavior. Relying on global variables makes the function’s behavior less predictable and harder to debug.
3. Initialization and Return Values:
   Even though I was returning $b from the add() function, in the addlots() method I never assigned this returned value to any variable. The use of the reference operator (&) means the changes should persist, but if $b is never properly initialized in the scope of addlots(), it may not contain what I expected after multiple function calls.

The Fix

I realized that the solution was to make sure $b was properly initialized before I started using it, and to rely more on returning the modified array rather than mixing global variable usage. Here’s how I refactored the code:

1. Initialize the Array:
   Before calling the function, initialize $b as an empty array.
2. Use Return Values Effectively:
   Even though I’m passing by reference, I’ll capture the returned array to make sure the value is updated properly.
3. Reduce Global Dependency:
   Where possible, I avoid using global and instead pass needed values directly to functions.

Here’s the corrected version:

public function add(&$b, $a)
{
  // Append the current element of $a to $b
  $b[] = current($a);
  echo current($b);
  next($a);
  // Return the modified array
  return $b;
}

public function addlots($a)
{
  $b = []; // Initialize the array
  $i = 0;
  $how_many = 5;
  
  while ($i < $how_many) {
    // Pass both the array by reference and the source array
    $this->add($b, $a);
    $i++;
  }

  // After processing, print the array
  print_r($b);
  return $b;
}

Now, when I call addlots($a) with a properly defined $a, the function accumulates values into $b and prints the expected result. This approach also makes the functions more modular and easier to test since they no longer rely on global state.

Final Thoughts

Working through this issue was a great learning experience. I discovered that when passing arrays between functions in PHP, proper initialization and careful management of references are key. Relying on global variables can often obscure the flow of data, making it harder to track where things are going wrong.

By refactoring the code to initialize the array and capture return values, I was able to get my functions to pass the array back as intended. I hope this walk-through helps you if you run into similar issues. Remember, clear data flow and avoiding unnecessary globals can save you a lot of headaches in the long run.