Add a string after dirname and slash using preg_replace

This should work for you:

<?php

    $path = "dirname/user folder/sample_file.png";

    echo $path = dirname($path) . "_trash/" . basename($path);

?>

Output:

dirname/user folder_trash/sample_file.png

EDIT:

You can search for the folder and then just replace it with the new one:

<?php

    $path = "dirname/user folder/another folder/sample_file.png";  

    $searchFolder = "/user folder/";
    $replaceFolder = "user folder_trash";

    echo $path = preg_replace($searchFolder, $replaceFolder, $path, 1); 

?>

Output:

dirname/user folder_trash/another folder/sample_file.png

EDIT 2:

IF the sting follows the same pattern every time this should work to change the second folder:

<?php

    $path = "dirname/user folder/another folder/another folder/sample_file.png";  

    $parts = explode("/", $path);
    $parts[1] = $parts[1] . "_trash";

    echo $path = implode("/", $parts);

?>

Output:

dirname/user folder_trash/another folder/another folder/sample_file.png