How do I read any request header in PHP

IF: you only need a single header, instead of all headers, the quickest method is:

<?php
// Replace XXXXXX_XXXX with the name of the header you need in UPPERCASE (and with '-' replaced by '_')
$headerStringValue = $_SERVER['HTTP_XXXXXX_XXXX'];

ELSE IF: you run PHP as an Apache module or, as of PHP 5.4, using FastCGI (simple method):

apache_request_headers()

<?php
$headers = apache_request_headers();

foreach ($headers as $header => $value) {
    echo "$header: $value <br />\n";
}

ELSE: In any other case, you can use (userland implementation):

<?php
function getRequestHeaders() {
    $headers = array();
    foreach($_SERVER as $key => $value) {
        if (substr($key, 0, 5) <> 'HTTP_') {
            continue;
        }
        $header = str_replace(' ', '-', ucwords(str_replace('_', ' ', strtolower(substr($key, 5)))));
        $headers[$header] = $value;
    }
    return $headers;
}

$headers = getRequestHeaders();

foreach ($headers as $header => $value) {
    echo "$header: $value <br />\n";
}

See Also:
getallheaders() – (PHP >= 5.4) cross platform edition Alias of apache_request_headers()
apache_response_headers() – Fetch all HTTP response headers.
headers_list() – Fetch a list of headers to be sent.