Array query - what is the most effective method to convert this array?

Like this you could:

$arr = array(

  "action: Added; quantity: 1; item_code: RNA1; product_name: Mens Organic T-shirt; colour: White; size: XL",

  "action: Subtracted; quantity: 7; item_code: RNC1; product_name: Kids Basic T-shirt; colour: Denim Blue; size: 3-4y",

  "action: Added; quantity: 20; item_code: RNV1; product_name: Gift Voucher; style: Mens; value: £20",

);

$outputarray = [];

foreach($arr as $key => $line){
  $items = explode(';', $line);
  foreach($items as $item){
      $property = trim(explode(':', $item)[0]);
      $value = trim(explode(':', $item)[1]);
      $outputarray[$key][$property] = $value;
  }
}

print_r($outputarray);

array(
    'action'    =>   '',
    'quantity'  =>   '',
    'item_code'     =>   '',
    'product_name'  =>   '',
    'colour'    =>   '',
    'size'      =>   '',
);

Is that ok?

Output of $outputarray:

Array ( [0] => Array ( [action] => Added [quantity] => 1 [item_code] => RNA1 [product_name] => Mens Organic T-shirt [colour] => White [size] => XL ) [1] => Array ( [action] => Subtracted [quantity] => 7 [item_code] => RNC1 [product_name] => Kids Basic T-shirt [colour] => Denim Blue [size] => 3-4y ) [2] => Array ( [action] => Added [quantity] => 20 [item_code] => RNV1 [product_name] => Gift Voucher [style] => Mens [value] => £20 ) )

One way to do it is by finding attributes and values in the strings using a regular expression, then merging them into key-value pairs with array_combine.

$items = array_map(function($item) {
    preg_match_all('/(\w+): ([^;]+)/', $item, $attributes);
    return array_combine($attributes[1], $attributes[2]);
}, $arr);