How to get data from xml that is sent to my script via http post?

This is how you loop trough XML nodes with PHP 4.0

<?php
$dom=domxml_open_file('file.xml');
// Or from a string
$dom=domxml_open_mem($xmlString);

$root=$dom->document_element;

$items=$root->get_elements_by_tagname('item');

// Loop trough childNodes
foreach ($items as $item) {
    $sku=$item->get_attribute('sku');

            // Your work here
}

echo $dom->dump_mem();
// Or to a file
$dom->dump_file('filename.xml');
?>

You can use file_get_contents() to fetch the XML string from the POST request. But I am not quite sure that "php://input" already works in PHP4.

$xmlData = file_get_contents("php://input");

In PHP4 there is the extension domxml for working with XML. But I would strongly recommend using PHP5 and the DOM extension.

To handle CSV data, I use str_getcsv()

while (($data = str_getcsv() !== FALSE)
{do something useful with $data}

For XML, I recommend the SimpleXML class as opposed to the XML Parser. Simple's not the most flexible, but it works just fine if all you're doing is reading in some data and saving it.

If the form data is just a text string, then you'd

$xml = simplexml_load_string($string)

And then

foreach ($xml->children() as $child)

{do something interesting}

Or you can just ditch the XML and dump it into an array with xml_parse_into_struct

$xmlparser = xml_parser_create();

xml_parse_into_struct($xmlparser,$xmldata,$values);

xml_parser_free($xmlparser);

If your vendor can send it in any way, why not send it as:
Valid PHP code, just send it as an array. If the Vendors script are running as PHP themself then this would be even easier as they can output valid PHP code using: var_export().

Also you could try using the XMLparsing functions found at the comments at this page: http://nl2.php.net/manual/en/function.xml-parse.php they all look pretty okay.

CSV would probably be simpler. You can use explode() to split the string (doc at https://www.php.net/explode).

For example, you can split the input into lines, then split each line into pieces on the commas:

// split up lines
$lines = explode("\n", $_POST['data']);
$data = array();
foreach ($lines as $row) {
    // split a line on the commas
    $prop = explode(',', $row);

    // append an element to the array that contains 
    // the properties as an associative array
    $data[] = array('sku' => $prop[0],
                    'price' => $prop[1],
                    'color' => $prop[2],
                    'shape' => $prop[3]);

}

Here, I'm assuming you want to convert the input into an array of elements, each element of the form:

array(4) {
  ["sku"] =>
  "434322",
  ["price"] =>
  "15.00",
  ["color"] => 
  "blue"
  ["shape"] =>
  "round"
}