Pop last value of array and use it as key for others items – php

I have a multidimensional array like

$inputArray = array(
    array(
        array('value31', 'value32'),
        array('value33', 'value34'),
        'value21'
    ),
    'value11'
);

I need to pop the last item from each item and considered it as the key for rest values. Output array should like,

$outputArray = array(
    'value11' => array(
        'value21' => array(
            array('value31', 'value32'),
            array('value33', 'value34'),
        )
    )
);

I tried this code, but it doesn’t work.

function convertArray(array $inputArray, $old_key = null) {
    $outputArray = [];
    $isMultiDimensional = count($inputArray) != count($inputArray, COUNT_RECURSIVE);
    $array_key = $isMultiDimensional ? array_pop($inputArray) : $old_key;

    foreach ($inputArray as $value) {
        $itemIsMultiDimensional = count($value) != count($value, COUNT_RECURSIVE);

        if (is_array($value) && $itemIsMultiDimensional) {
            $outputArray = array_merge($outputArray, convertArray($value, $array_key));
        } else {
            $outputArray[$array_key][] = $value;
        }
    }

    return $outputArray;
}