I think you can use array_reduce() function. For example:

   $multi= array(0 => array('t1' => 'test1'),1 => array('t2' => 'test2'),2 => array('t3' => 'test3'),3 => array('t4' => 'test4'));
   $single= array_reduce($multi, 'array_merge', array());
   print_r($single);  //Outputs the reduced aray

This single line would do that:

$array = array_column($array, 'plan');

The first argument is an array | The second argument is an array key.

For details, go to official documentation: https://www.php.net/manual/en/function.array-column.php.

Use array_column()

Returns an array of values representing a single column from the input array.

<?php

$user_array = array(
                  0 => array('user_id' => 1, 'name' => 'Bob'),
                  1 => array('user_id' => 2, 'name' => 'John'),
                  2 => array('user_id' => 3, 'name' => 'Mary')
              );

$users = array_column($user_array, 'user_id');

print_r($users);

Output :

Array
(
   [0] => 1
   [1] => 2
   [2] => 3
)

$result = call_user_func_array('array_merge', $array);

<?php
$array = array(
    'one' => 'one_value',
    'two' => array
        (
            'four' => 'four_value',
            'five' => 'five_value'
        ),

    'three' => array
        (
            'six' => array
                (
                    'seven' => 'seven_value'
                )

        )
);

function flatten($array, $prefix = '') {
    $arr = array();
    foreach($array as $k => $v) {
        if(is_array($v)) {
            $arr = array_merge($arr, flatten($v, $prefix . $k . '-'));
        }
        else{
            $arr[$prefix . $k] = $v;
        }
    }
    return $arr;
}

var_dump(flatten($array));

//output:
//array(4) {
//  ["one"]=>
//  string(9) "one_value"
//  ["two-four"]=>
//  string(10) "four_value"
//  ["two-five"]=>
//  string(10) "five_value"
//  ["three-six-seven"]=>
//  string(11) "seven_value"
//}

Running example

Easy:

$flattened = [];
foreach ($mdarray as $arr) {
    foreach ($arr as $val) {
        $flattened[] = $val;
    }
}

What the above code does is it iterates through the multi-dimensional array, gets the value of each subarray, and adds its elements to $flattened.

If you're using PHP 5.6+, this code is simpler:

$flattened = array_merge(...$mdarray);

array_reduce($array, 'array_merge', array())

Example:

$a = array(array(1, 2, 3), array(4, 5, 6));
$result = array_reduce($a, 'array_merge', array());

Result:

array[1, 2, 3, 4, 5, 6];

It looks like you want all sub array values to be in the single array.

$singleArray = [];
foreach($multiarray as $array) {
    $singleArray = array_merge($singleArray, array_values($array));
}

This may contain some values as a duplicate. To clean them up you can do

$uniqueValues = array_unique($singleArray);

This assumes that you will have more than one array to process and simply sorts your elements into a new, formatted array.

<?php
$results = array(
    array(
        'city' => 'xyz',
        'name' => array( 'Xyz1', 'xyz2' ),
        'email' => array( 'xyz1@gmail.com', 'xyz2@gmail.com' ),
        'phone' => array( '23423423-1', '23423423-2' ),
        'address' => array( 'xyz1', 'xyz2' ),
        'dis' => 'xyz'
    )
);

$formatted_results = array();


foreach( $results as $result ) {

    foreach( array(0,1) as $key ) {
        $formatted_results[][$key] = array(
        'city' => $result['city'],
        'name' => $result['name'][$key],
        'email' => $result['email'][$key],
        'phone' => $result['phone'][$key],
        'address' => $result['address'][$key],
        'dis' => $result['dis']
        );
    }

}

echo "<pre>";

print_r($formatted_results);

This will output

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [city] => xyz
                    [name] => Xyz1
                    [email] => xyz1@gmail.com
                    [phone] => 23423423-1
                    [address] => xyz1
                    [dis] => xyz
                )

        )

    [1] => Array
        (
            [1] => Array
                (
                    [city] => xyz
                    [name] => xyz2
                    [email] => xyz2@gmail.com
                    [phone] => 23423423-2
                    [address] => xyz2
                    [dis] => xyz
                )

        )

)

You can then access your new values like this:

foreach( $formatted_results as $value ) {
    $r1 = $value[0];
    $r2 = $value[1];
}

Or if you only have the single array:

$r1 = $formatted_results[0][0];
$r2 = $formatted_results[0][1];

As long as 0 is the key name and 1 is the value, then just create a single dimension indexed on the first element and build the query string:

$string = http_build_query(array_column($array, 1, 0));