Setting dynamic session variables(arrays) with a prefix and looping with foreach

According to the Joomla JSession documentation the JSession class does provide a getIterator method returning an ArrayIterator.

As a reusable approach, you could implement your own FilterIterator class, only iterating over items having a specific prefix and optionally stripping the prefix from keys.

In your code you get the iterator by

$sessionArrayIter = $session->getIterator();

Since I do not well know Joomla and do not have any installation running, I will spoof that part:

$sessionArray     = ['aa_test1' => 1, 'bb_test2' => 2, 'aa_test3' => 3, 'cc_test4' => 4];
$sessionArrayIter = new ArrayIterator($sessionArray);

Class Implementation

Then we implement the PrefixFilterIterator class extending PHP's abstract FilterIterator class.

class PrefixFilterIterator extends FilterIterator
{
  private
    $_prefix,
    $_prefixLength,
    $_strip_prefix
  ;

  public function __construct(Iterator $iterator, string $prefix, bool $strip_prefix = false)
  {
    parent::__construct($iterator);
    $this->set_prefix($prefix, $strip_prefix);
  }

  public function set_prefix(string $prefix, ?bool $strip_prefix = null) : void
  {
    $this->_prefix       = $prefix;
    $this->_prefixLength = strlen($prefix);
    if(null !== $strip_prefix)
      $this->_strip_prefix = $strip_prefix;
  }

  // conditionally remove prefix from key
  public function key()  /* : mixed scalar */
  {
    return $this->_strip_prefix ? substr(parent::key(), $this->_prefixLength) : parent::key();
  }

  // accept prefixed items only
  public function accept() : bool
  {
    return 0 === strpos(parent::key(), $this->_prefix);
  }
}

Usage

To iterate over the filtered items, we create a new instance of our iterator.

$prefixIter = new PrefixFilterIterator($sessionArrayIter, 'aa_', true);

foreach ($prefixIter as $k => $v)
  echo "$k => $v", PHP_EOL;

Output

test1 => 1
test3 => 3

live demo

Remarks, Limits, ToDo:

The code above runs on PHP >= 7.1

To support PHP 7.0, type hints must be adapted. :void is not supported in PHP<7.1 and must be removed, likewise ?bool must be changed into bool.

This is a simple implementation focussing the problem in the question to reduce the 'noise' in the answer. mbstring is a non-default extension to PHP. Therefore I did not use multibyte string functions. However, array keys may include multibyte charactersets. To support such keys, a conditional implementation of some string function wrappers would be required, using appropriate functions if they are installed. The preg_* functions with the u modifier could be an alternative to support multibyte unicode keys.