<?php

$foo = "foo.txt da\ code.txt bar.txt";

$array1 = explode(' ', $foo);
$array2 = array();
$appendNext = false;
foreach($array1 as $elem)
{
    if ($appendNext)
    {
        array_push($array2, array_pop($array2) . ' ' . $elem);
    }
    else
    {
        $array2[] = $elem;
    }
    $appendNext = (substr($elem, -1) === '\\');
}
var_dump($array2);

array(3) {
  [0]=>
  string(7) "foo.txt"
  [1]=>
  string(12) "da\ code.txt"
  [2]=>
  string(7) "bar.txt"
}