The $_FILES["FILE_NAME"]['error'] (where "FILE_NAME" is the value of the name attribute of the file input, present in your form) might contain one of the following values:

1. UPLOAD_ERR_OK - There is no error, the file uploaded with success.
2. UPLOAD_ERR_INI_SIZE - The uploaded file exceeds the upload_max_filesize directive in php.ini.
3. UPLOAD_ERR_PARTIAL - The uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form.
4. UPLOAD_ERR_NO_FILE - No file was uploaded.
5. UPLOAD_ERR_NO_TMP_DIR - Missing a temporary folder. (From PHP 5.0.3).
6. UPLOAD_ERR_CANT_WRITE - Failed to write file to disk. (From PHP 5.1.0).
7. UPLOAD_ERR_EXTENSION - A PHP extension stopped the file upload. (From PHP 5.2.0).

An basic way to check for the errors, is as follows:

<?php
$fileError = $_FILES["FILE_NAME"]["error"]; // where FILE_NAME is the name attribute of the file input in your form
switch($fileError) {
    case UPLOAD_ERR_INI_SIZE:
        // Exceeds max size in php.ini
        break;
    case UPLOAD_ERR_PARTIAL:
        // Exceeds max size in html form
        break;
    case UPLOAD_ERR_NO_FILE:
        // No file was uploaded
        break;
    case UPLOAD_ERR_NO_TMP_DIR:
        // No /tmp dir to write to
        break;
    case UPLOAD_ERR_CANT_WRITE:
        // Error writing to disk
        break;
    default:
        // No error was faced! Phew!
        break;
}

Data from a POST request is stored in the superglobal $_POST in the form of an associative array.

Note that accessing a non-existent array item generates a notice, so existence should always be checked with the isset() or empty() functions, or the null coalesce operator.

Example:

$from = isset($_POST["name"]) ? $_POST["name"] : "NO NAME";
$message = isset($_POST["message"]) ? $_POST["message"] : "NO MESSAGE";

echo "Message from $from: $message";

$from = $_POST["name"] ?? "NO NAME";
$message = $_POST["message"] ?? "NO MESSAGE";

echo "Message from $from: $message";

Data from a GET request is stored in the superglobal $_GET in the form of an associative array.

Note that accessing a non-existent array item generates a notice, so existence should always be checked with the isset() or empty() functions, or the null coalesce operator.

Example: (for URL /topics.php?author=alice&topic=php)

$author = isset($_GET["author"]) ? $_GET["author"] : "NO AUTHOR";
$topic = isset($_GET["topic"]) ? $_GET["topic"] : "NO TOPIC";

echo "Showing posts from $author about $topic";

$author = $_GET["author"] ?? "NO AUTHOR";
$topic = $_GET["topic"] ?? "NO TOPIC";

echo "Showing posts from $author about $topic";

Usually data sent in a POST request is structured key/value pairs with a MIME type of application/x-www-form-urlencoded. However many applications such as web services require raw data, often in XML or JSON format, to be sent instead. This data can be read using one of two methods.

php://input is a stream that provides access to the raw request body.

$rawdata = file_get_contents("php://input");
// Let's say we got JSON
$decoded = json_decode($rawdata);

$rawdata = $HTTP_RAW_POST_DATA;
// Or maybe we get XML
$decoded = simplexml_load_string($rawdata);

Note that neither of these methods are available when the content type is set to multipart/form-data, which is used for file uploads.

PHP provides support for the HTTP PUT method used by some clients to store files on a server. PUT requests are much simpler than a file upload using POST requests and they look something like this:

PUT /path/filename.html HTTP/1.1

Into your PHP code you would then do something like this:

<?php
/* PUT data comes in on the stdin stream */
$putdata = fopen("php://input", "r");

/* Open a file for writing */
$fp = fopen("putfile.ext", "w");

/* Read the data 1 KB at a time
   and write to the file */
while ($data = fread($putdata, 1024))
  fwrite($fp, $data);

/* Close the streams */
fclose($fp);
fclose($putdata);
?>

Also here you can read interesting SO question/answers about receiving file via HTTP PUT.

Usually, an HTML form element submitted to PHP results in a single value. For example:

<pre>
<?php print_r($_POST);?>
</pre>
<form method="post">
    <input type="hidden" name="foo" value="bar"/>
    <button type="submit">Submit</button>
</form>

This results in the following output:

Array
(
    [foo] => bar
)

However, there may be cases where you want to pass an array of values. This can be done by adding a PHP-like suffix to the name of the HTML elements:

<pre>
<?php print_r($_POST);?>
</pre>
<form method="post">
    <input type="hidden" name="foo[]" value="bar"/>
    <input type="hidden" name="foo[]" value="baz"/>
    <button type="submit">Submit</button>
</form>

This results in the following output:

Array
(
    [foo] => Array
        (
            [0] => bar
            [1] => baz
        )

)

You can also specify the array indices, as either numbers or strings:

<pre>
<?php print_r($_POST);?>
</pre>
<form method="post">
    <input type="hidden" name="foo[42]" value="bar"/>
    <input type="hidden" name="foo[foo]" value="baz"/>
    <button type="submit">Submit</button>
</form>

Which returns this output:

Array
(
    [foo] => Array
        (
            [42] => bar
            [foo] => baz
        )

)

This technique can be used to avoid post-processing loops over the $_POST array, making your code leaner and more concise.