Here are a few considerations:

• You might rename $value to $subarray to be more literal about the data that it holds.

• Because there is no need to modify the subarray, you can access the lone value by calling current() instead of array_pop().

• You can combine your nested conditional logic into one and eliminate the declaration of $data by assigning directly to $temp[$groupValue][]

Just before the return:

if (!$preserveSubArrays && count(key]) == 1) {
    $temp[$groupValue][] = current(key]);
} else {
    $temp[$groupValue][] = key];
}

I guess I should also point out that your function makes a few assumptions about the data being passed to it. There are a few ways to elicit Notices/Warnings from this function by passing an uncooperative array of data or grouping value. This may be a "tinfoil hat" observation for the scope of your question, but I thought I'd acknowledge it.

Generally, it looks likes a good cover-all function for the many StackOverflow questions that seek what this function can provide.

p.s. In a previous comment that I have since deleted, I suggested replacing unset() with another function. This was before I fully understood what this function was doing. I toyed with an approach that used array_diff_key(), but there was no advantage. unset() is sure to be the most efficient way to modify the subarray (when necessary).

Years later "re-review":

• Declare the argument and return data types to make your method a bit more strict and easier to maintain.
• Use square brace syntax for declaring an array -- it is just more concise and consistent with other square brace syntax used for pushing elements.
• if(!array_key_exists($groupValue, $temp)) { $temp[$groupValue] = array(); } is unnecessary because in php you don't need to instantiate an array before you push elements into it using [] syntax. BUT, it makes good sense to check if a given row actually contains the grouping column -- else you will be trying to access an undefined key.
• Obey PSR-12 coding recommendations.
• Endeavor to remove all single-use variables unless they improve the readability/comprehensibility of your script.
• When accessing the potentially unset grouping values, reliably find it using the undisturbed original array (not the copy being iterated).

Suggested rewrite:

public function groupArray(
    array $arr,
    string|int $group,
    bool $preserveGroupKey = false,
    bool $preserveSubArrays = false
): array {
    $result = [];
    foreach (key => $row) {
        if (!key_exists($group, $row)) {
            throw new Exception("Could not group by $group on row with key $key");
        }
        if (!$preserveGroupKey) {
            unset(group]);
        }
        $result[key][$group]][] = !$preserveSubArrays && count($row) === 1
            ? current(row;
    }
    return $result;
}

Best way, if you have control over building the initial array, is just set things up like that at the start as you add entries.

If not then build a temporary array to sort:

foreach ($input_arr as $key => &$entry) {
    $level_arr[$entry['level']][entry;
}

Leaves you with the form you wanted and everything referenced together.

Build the array like that in the first place though if at all possible.

Here is a solution with demo. Might help you:

// $myArr is your origional array.
$result_arr = [];

array_walk($myArr,function($v,$k) use (&$result_arr){
   $result_arr[key($v)][] = $v[key($v)]; 
});

print_r($result_arr);

Click Here for Demo

What about this:

$Show = array();
foreach ($result_radio as $Station => $Show){
$Show [$result_radio[$Station]['StationID']][] = $Show; } 

It should do exactly what you want.

$class_array = array();
foreach ($students_array as $sa) {
    $class_array[$sa['class']][] = array('sid' => $sa['sid']);
}

Copy<?php
$grouped = array();
foreach ($yourData as $item) {
  // copy item to grouped
  $grouped[$item['team']][] = $item;
}
var_dump($grouped);

Demo

foreach($files as $file){
    $path_parts = pathinfo($file);
    $date = date ("jS \of F,  Y", filemtime($file));
    if(isset($dateUploaded[$date])) {
          $dateUploaded[$date][] = $path_parts['filename'];
    } else {
          $dateUploaded[$date] = array($path_parts['filename']);
    }
}

I hope this helps

public function array_group_by($array, $key) {

    if (is_null($key)) return $array;
    $result = array();

    foreach ($array as $item) {
        $group_key = $item[$key];
        if (!array_key_exists($group_key, $result)) {
            $result[$group_key] = array();
        }
        $result[$group_key][] = $item;
    }

    return $result;
}

so you can call array_group_by($proplist, 'state'); which should give you the required result as an array.

var_dump(array_group_by($proplist,'state');

should output

array (size=3)
  'GA' => 
    array (size=2)
      0 => 
        array (size=5)
          'name' => string 'Store #1' (length=8)
          'address' => string '123 Example St' (length=14)
          'city' => string 'Smyrna' (length=6)
          'state' => string 'GA' (length=2)
          'zip' => string '11111' (length=5)
      1 => 
        array (size=5)
          'name' => string 'Store #4' (length=8)
          'address' => string '111 Someplace Rd' (length=16)
          'city' => string 'Atlanta' (length=7)
          'state' => string 'GA' (length=2)
          'zip' => string '22222' (length=5)
  'VA' => 
    array (size=1)
      0 => 
        array (size=5)
          'name' => string 'Store #2' (length=8)
          'address' => string '666 Anywhere Rd' (length=15)
          'city' => string 'Bristol' (length=7)
          'state' => string 'VA' (length=2)
          'zip' => string '33333' (length=5)
  'NC' => 
    array (size=1)
      0 => 
        array (size=5)
          'name' => string 'Store #3' (length=8)
          'address' => string '123 Any Street' (length=14)
          'city' => string 'Bristol' (length=7)
          'state' => string 'NC' (length=2)
          'zip' => string '44444' (length=5)

You can also find a working example here https://eval.in/98276

Roughly like this:

$groupedItems = array();

foreach($data as $item)
{
    $groupedItems[$item['property_id']][] = $item;
}

// See @Lepanto's comment below, this resets the keys to match output OP required: 
$groupedItems = array_values($groupedItems);