<?php

$foo = "foo.txt da\ code.txt bar.txt";
$foos = array();
$char = 0;
$index = 0;
$lookback = '';

while ($char < strlen($foo)) {
    $lookback = $foo{$char-4} . $foo{$char-3} . $foo{$char-2} . $foo{$char-1} . $foo{$char};

    if ($lookback == '.txt ') $index++;

    $foos[$index] .= $foo{$char++};
}

print_r(array_map('trim', $foos));

?>

Array
(
    [0] => foo.txt
    [1] => da\ code.txt
    [2] => bar.txt
)