Static class to facilitate looking up request headers. The class allows you to look up request headers case insensitively.

RequestHeaders.php

<?php

/**
 * Class RequestHeaders
 *
 * Static class to facilitate looking up request headers.
 */
class RequestHeaders
{
    /**
     * Headers from apache_request_headers with keys normalize to lowercase.
     *
     * @var array
     */
    static private $headers;

    /**
     * Return the value of the header with the given field name. If not set,
     * return null.
     *
     * @param string $headerField
     * @return string|null
     */
    public static function getHeader($headerField)
    {
        if (!isset(self::$headers)) {
            self::readHeaders();
        }

        // The keys in the headers array are normalized to lowercase, so
        // normalize the argument to match.
        $headerField = strtolower($headerField);

        if (isset(self::$headers[$headerField])) {
            return self::$headers[$headerField];
        }
        return null;
    }

    /**
     * Store headers returned by apache_request_headers() with the field
     * names normalized to lowercase.
     */
    private static function readHeaders()
    {
        // Copy the list of headers and normalize the case of the field names.
        $headers = apache_request_headers();
        self::$headers = array();
        foreach ($headers as $k => $v) {
            self::$headers[strtolower($k)] = $v;
        }
    }

}