var ajax_object // Your object

.ajax({
    type: "POST",
    url: "tofile.php",
    data: ajax_object,
});

// File: tofile.php

$ajax_object // Object, which You have passed using AJAX

ob_start();
print_r("<pre>".print_r($ajax_object, true)."</pre>");
$var = ob_get_contents();
ob_end_clean();
$fp = fopen('somefile.htm', 'w');
fputs($fp, $var);
fclose($fp);

Some_Object Object
(
    [some_element] => 123
    [some_array] => Array
        (
            [element1] => 456
            [element2] => 789
            [element3] => 012
        )
)