How to order a PHP multidimensional array by one of its subarray's key

You need to preserve the outer keys, use uasort(). I prefer the spaceship (3-way comparison) operator between two key() calls.

Code: (Demo)

$options = [
    'option_3' => [
        'Rio' => '18',
    ],
    'option_4' => [
        'Tokyo' => '20',
    ],
    'option_5' => [
        'Berlim' => '23',
    ]
];

uasort($options, function ($a, $b) {
    return key($a) <=> key($b);
});
var_export($options);

Output:

array (
  'option_5' => 
  array (
    'Berlim' => '23',
  ),
  'option_3' => 
  array (
    'Rio' => '18',
  ),
  'option_4' => 
  array (
    'Tokyo' => '20',
  ),
)

From PHP7.4+, You can use arrow syntax inside custom functions. (Demo)

uasort($options, fn($a, $b) => key($a) <=> key($b));

Preparing for an array_multisort() call is prohibitively hideous, IMO. (Demo)

array_multisort(array_keys(array_merge(...array_values($options))), $options);

Firstly, create a single array for each city and push them into a new array like that:

$options = [
    'option_3' => [
        'Rio' => '18',
    ],
    'option_4' => [
        'Tokyo' => '20',
    ],
    'option_5' => [
        'Berlim' => '23',
    ]
];

$newOptions = [];

foreach($options as $key => $option) {
    array_push($newOptions, [
        'id' => $key,
        'city' => key($option),
        'price' => reset($option)
    ]);
}

After, just sort it alphabetically by the value of each 'city' using usort()

usort($newOptions, function($a, $b) {
    return strcmp($a['city'], $b['city']);
});

Finally, you can simply do a foreach into $newOptions and echo whatever you want based on keys 'id', 'city' and 'price'. Example:

foreach($newOptions as $option)

<tr>
    <td><?php echo $option['id'];?></td>
    <td><?php echo $option['city']?></td>
    <td><?php echo $option['price']?></td>
    <td><button type='submit'>Remove</button></td>
</tr>

Ok, this sort is little trickier as we need to preserve the parent key for each city as well. So, we go as follows:

• Modify the current array to have an extra parent_key in it's subarray which would look like:

  'option_3' => array (
      'Rio' => '18',
      'parent_key' => 'option_3'
  )
  

• Now, we use usort() to compare the 2 subarrays via name through strcmp() and return the result.

• We again iterate through them and use array_keys() to iterate over the data set. We do this as we are going to restore the parent key back without running into concurrent modification issue while iterating over the same array.

Code:

<?php

$data = array (
            'option_3' => array (
                'Rio' => '18',
            ),
            'option_4' => array (
                'Tokyo' => '20',
            ),
            'option_5' => array (
                'Berlim' => '23',
            )
        );


foreach($data as $key => &$value){
    $value['parent_key'] = $key;
}


usort($data,function($val1,$val2){
    $name1 = array_keys($val1)[0];
    $name2 = array_keys($val2)[0];
    return strcmp($name1,$name2); 
});

foreach(array_keys($data) as $index){
    $value = $data[$index];
    $parent_key = $value['parent_key'];
    unset($value['parent_key']); // unset parent key now as it is no longer needed
    $data[$parent_key] = $value;
    unset($data[$index]); // unset numeric index modified by usort()
}


print_r($data);

Demo: https://3v4l.org/q0YFk

Update: Current result does not hold extra keys like city etc, but it is easy to figure that out as a city name while doing a foreach in key-value fashion.

You can accomplish that in many ways, here is my way Demo, hope it works for you.

$arr = array (
    'option_3' => array (
        'Rio' => '18',
    ),
    'option_4' => array (
        'Tokyo' => '20',
    ),
    'option_5' => array (
        'Berlim' => '23',
    )
);

$cities = [];

foreach($arr as $key=>$val){
    foreach($val as $key2=>$city){

        $option = explode('_', $key);
        $cities[$key2] = $city.'_'.$option[1];
    }
}
ksort($cities);

$res = [];
foreach($cities as $key=>$val){
    $temp = [];
    $temp['city'] = $key;
    $val_opt = explode('_', $val);
    $temp['value'] = $val_opt[0];
    $temp['option'] = $val_opt[1];

    $res[] = $temp;
}

print_r($res);

Output:

Array
(
    [0] => Array
        (
            [city] => Berlim
            [value] => 23
            [option] => 5
        )

    [1] => Array
        (
            [city] => Rio
            [value] => 18
            [option] => 3
        )

    [2] => Array
        (
            [city] => Tokyo
            [value] => 20
            [option] => 4
        )

)