<?php

function flattenArray($arr) {
	$output = [];
	
	foreach ($arr as $key => $value) {
		if (is_array($value)) {
			foreach(flattenArray($value) as $flattenKey => $flattenValue) {
				$output["${key}.${flattenKey}"] = $flattenValue;
			}
		} else {
			$output[$key] = $value;
		}
	}
	
	return $output;
}

$arr = [
    'applicant' => [
        'user' => [
            'username' => true,
            'password' => true,
            'data' => [
                'value' => true,
                'anotherValue' => true
            ]
        ]
    ]
];

var_dump(flattenArray($arr));