How to add array key to child array in PHP

What you are trying to do is impossible but there are ways to achieve something similar.

Key-Value Pairs

You can not have two same indices in one array, but you can have an array of key-value pairs, if you change the line

$singleArray[] = $value;

with

$singleArray[] = ["key" => $k, "value" => $value];

you'll get this output:

Array
(
    [0] => Array
        (
            [key] => 296
            [value] => 02/22/2020
        )

    [1] => Array
        (
            [key] => 296
            [value] => 03/03/2020
        )

    [2] => Array
        (
            [key] => 297
            [value] => 02/22/2020
        )

    [3] => Array
        (
            [key] => 300
            [value] => 02/21/2020
        )

)

Key-Value Pairs (Object Oriented)

A better way is to use a specific data structure for this key-value pairs, for example use

class KeyValue {
    public $key;
    public $value;
    function __construct ($key, $value) {
        $this->key = $key;
        $this->value = $value;
    }
}

and then use

$singleArray[] = new KeyValue($k, $value); 

Or even more sophisticated, you can use

class UserDate {
    public $user;
    public $date;
    function __construct ($user, $date) {
        $this->user = $user;
        $this->date = $date;
    }
}

Array of Arrays

Which you done it already in $tourDates. For displaying this data you're gonna need two nested loops as you used to convert it to $singleArray.

foreach ($tourDates as $k=> $childArray) 
{ 
    //echo $k;
    foreach ($childArray as $ky=> $value) 
    {
        echo $k."\t".$value."\r\n"; 
    } 
}

Instead of looping over $tourDates, maybe loop over $dateARRa

<?php
$dateARRa = array( 
    array( 
        'user_id'   => '296',
        'tour_date' => array( 
           'dubai'     => '02/22/2020', 
           'Melbourne' => '03/03/2020'
        ) 
    ),
    array ( 
        'user_id'   => '297',
        'tour_date' => array(
            'Singapore' => '02/22/2020'
        )
    ),
    array( 
        'user_id'   => '300',
        'tour_date' => array(
            'Sydeny' => '02/21/2020'
        )
    ),
);


$singleArray = []; 

// first we loop over $dateARRa
foreach ( $dateARRa as $k ) { 

    // we check that the variables we need exist
    if ( !empty( $dateARRa[$k]['user_id'] ) && ! empty( $dateARRa[$k]['tour_date'] ) ) ) {

        // Then, because we want to loop over an internal array, we begin that here. 
        // This is inefficient and should be avoided. But perfection is the enemy of good-enough.
        foreach( $dateARRa[$k]['tour_date'] as $tour_date ) {

            // we check if the user_id index exists on the `$singleArray`
            if ( ! empty( $singleArray[ $dateARRa[$k]['user_id'] ] ) ) {

                /* if it exists we can add a new tour-date to the array
                   note that here $tour_date would be equal to 'dubai', 'Melbourne', 'Singapore', or 'Sydeny', but $dateARRa[$k]['tour_date'][ $tour_date ] would be the date-value regardless of the name of the index (which is in this case the location-name)
                 */
                $singleArray[ $dateARRa[$k]['user_id'] ][] = $dateARRa[$k]['tour_date'][ $tour_date ];

                /* an alternative could be to push a new array-item containing both the location name and the date, but i'll leave that commented out because it's tangental
                   $singleArray[ $dateARRa[$k]['user_id'] ][] = [ 'location' => $tour_date, 'date' => $dateARRa[$k]['tour_date'][ $tour_date ]; 
                 */

            } else {

                /* and if it doesn't exist we create the index, using the user-id and an array with a single-index
                 */
                $singleArray[ $dateARRa[$k]['user_id'] ] = array( $tour_date );
            }

        }
    }
    $singleArray[ $dateARRa[ $k ][ 'user_id' ] ] = $dateARRa[ $k ][ 'user_id' ];
}
echo "<pre>";print_r($singleArray);

// expected output
Array
(
    [296] => [ 02/22/2020, 03/03/2020 ]
    [297] => [ 02/22/2020 ]
    [300] => [ 02/21/2020 ]
)

expected output

Array
(
    [296] => [ 02/22/2020, 03/03/2020 ]
    [297] => [ 02/22/2020 ]
    [300] => [ 02/21/2020 ]
)