<?php

$data = '2018-04-20';
$hora = '07:40:30';

echo date('Y-m-d H:i:s', strtotime("{$data} {$hora}")), PHP_EOL;
echo date('Y-m-d H:i:s', strtotime('{$data} {$hora}')), PHP_EOL;

echo "{$data} {$hora}", PHP_EOL;
echo '{$data} {$hora}', PHP_EOL;