here is simple recursive function:
function tree2array($input, &$output, $prefix = '')
{
foreach ($input as $i => $v)
if (is_array($v))
tree2array($v, $output, $prefix.'/'.$i);
else
$output[] = $prefix.'/'.$v;
}
usage:
tree2array($array_1, $array2);
output:
print_r($array2);
Array (
[0] => /f1/f2/file1.php
[1] => /f1/f2/file2.php
[2] => /f1/f3/file3.php
[3] => /f1/f3/file4.php
[4] => /f1/f4/fol5/fileAA.php
[5] => /f1/f4/fol5/fileBB.php
[6] => /f1/f4/fileDD.php
[7] => /f2/f2/file1.php
[8] => /f2/f2/file2.php
[9] => /f2/f3/file3.php )