在该数组中按值循环数组分组索引（PHP）

Question

I wish to create a list of items from an array, grouped by a value within that array.

Take this array:

$people = array(
    0 => array(
         "Forename" => "Jim",
         "Surname"  => "Smith"
    ),
    1 => array(
         "Forename" => "Mike",
         "Surname"  => "Johnson"
    ),
    2 => array(
         "Forename" => "Kim",
         "Surname"  => "Smith"
    ),
    3 => array(
         "Forename" => "Paul",
         "Surname"  => "Jones"
    )
);

Specifically I'd like to run a foreach on $people, grouping them by unique surname. i.e. the desired output would be:

<select>
    <optgroup label="Smith">
        <option>Jim</option>
        <option>Kim</option>
    </optgroup>
    <optgroup label="Johnson">
        <option>Mike</option>
    </optgroup>
    <optgroup label="Jones">
        <option>Paul</option>
    </optgroup>
</select>

I'm struggling to come up with anything vaguely efficient and the Google gods aren't watching over me today :( What's the best approach for such a use-case in PHP?

Answer 1

$surnames = array();
foreach($people as $person) {
    $surnames[$person['surname']][] = $person;
}

This code stores all persons in an array grouped by their surnames.

The resulting array:

array(
 'smith' => array(
   0 => array(
     "Forename" => "Jim",
     "Surname"  => "Smith"
   ),
   1 => array(
     "Forename" => "Kim",
     "Surname"  => "Smith"
   )
 ),
 'jones' => array(
   0 => array(
     "Forename" => "Paul",
     "Surname"  => "Jones"
   )
 )
)