Test file to analyze the best algorithm for parsePriority.. ;-) Modified to include "Importance" header and string based values.

testParsePriority.php

<?php
define ( 'HIGH_PRIORITY', 1 );
define ( 'NORMAL_PRIORITY', 2 );
define ( 'LOW_PRIORITY', 3 );
define ( 'NO_PRIORITY', 0 );

echo "Time to beat: 0.027ms\n";
test ( 'StrPos', 'strposversion' );
test ( 'PregMatch', 'pregmatchversion' );

function test($name, $func) {
	$fastest = 1;
	$total = 0;
	for($i = 0; $i < 10000; $i ++) {
		$starttime = microtime ( true );
		foreach ( array (
				'X-Priority: isNAN' => NO_PRIORITY, // input => output
				'X-Priority: 1' => LOW_PRIORITY,
				'X-Priority: 2' => LOW_PRIORITY,
				'X-Priority: 3' => NORMAL_PRIORITY,
				'X-Priority: 4' => NORMAL_PRIORITY,
				'X-Priority: 5' => HIGH_PRIORITY,
				'X-Priority: 6' => HIGH_PRIORITY,
				'No priority set' => NO_PRIORITY,
				'Priority: normal' => NORMAL_PRIORITY,
				'xyz-priority: high' => HIGH_PRIORITY,
				'Priority: high' => HIGH_PRIORITY,
				'priority: low' => LOW_PRIORITY,
				'x-priority: 1000' => LOW_PRIORITY, // only matches first 1, not the full 1000
				'priority: 3' => NORMAL_PRIORITY,
				'IPM-Importance: low' => LOW_PRIORITY,
				'My-Importance: URGENT' => HIGH_PRIORITY,
				'Urgency: High' => NO_PRIORITY, //urgency doesn't match.. maybe it should?
				'X-Importance: 5' => HIGH_PRIORITY,
				'' => NO_PRIORITY 
		) as $priority => $response ) {
			if (! (call_user_func ( $func, h ( $priority ) ) == $response))
				throw new Exception ( "Algorithm mistake: $priority should return $response!" );
		}
		$time = (microtime ( true ) - $starttime);
		if ($time < $fastest)
			$fastest = $time;
		$total += $time;
	}
	print $name . ': ' . number_format ( $fastest * 1000, 3 ) . "ms, total: " . number_format ( $total, 3 ) . "s\n";
}

// runs in  0.350ms, on my laptop, seems to be pretty useful
function pregmatchversion($header = null) {
	if (! $header)
		return 0;
		// Test for normal "X-Priority: INT" style header & stringy version.
		// Allows for Importance possibility.
	if (preg_match ( '/priority: (\d|\w)/i', $header, $matching_char ) 
   || preg_match ( '/importance: (\d|\w)/i', $header, $matching_char )) {
		
		switch ($matching_char[1]) {
			case 'h' : 
			case 'H' :// high
			case 'u':
			case 'U': //Urgent
			case 6 :
			case 5 :
				return 1;
			case 'n' : // normal
			case 'N' :
			case 4 :
			case 3 :
				return 2;
			case 'l' : // low
			case 'L' :
			case 2 :
			case 1 :
				return 3;
		}
	}
	
	return 0;
}

// runs in 0.216ms, on my laptop, obviously VERY rigidly defined..
function strposversion($header = null) {
	if (! $header)
		return 0;
		// Get the number of characters before the start of the header we are after.
	if ($p_header = stripos ( $header, 'priority:' )) {
		// From start of the priority header, capture a single char to indicate priority
		if (! $priority_char = substr ( $header, ($p_header + 10), 1 ))// "Priority: + space" is 10 characters
			return 0;
		
	} elseif ($p_header = stripos ( $header, 'importance:' )) {
		if (! $priority_char = substr ( $header, ($p_header + 12), 1 ))// "Importance: + space" is 12 chars.
			return 0;
	}else{
		return 0;
	}
	
	switch ($priority_char) {
		case 'u':
		case 'U': //Urgent
		case 'h' ://High
		case 'H' : // was faster to include caps in switch than call strtolower on all.
		case 6 :
		case 5 :
			return 1;
		case 'n' : //Normal priority (or no)
		case 'N' :
		case 4 :
		case 3 :
			return 2;
		case 'l' : //Low priority
		case 'L' :
		case 2 :
		case 1 :
			return 3;
	}
	return 0;
}

function h($setPriority = "") {
	return <<<HEADER
                                                                             
Delivered-To: clonemeagain@gmail.com
Received: by 10.69.18.42 with SMTP id gj10csp88238pbd;
        Fri, 20 Dec 2013 10:08:25 -0800 (PST)
X-Received: by 10.224.13.80 with SMTP id b16mr16256982qaa.73.1387562904239;
        Fri, 20 Dec 2013 10:08:24 -0800 (PST)
Return-Path: <noreply@github.com>
Received: from github-smtp2a-ext-cp1-prd.iad.github.net (github-smtp2-ext5.iad.github.net. [192.30.252.196])
        by mx.google.com with ESMTPS id k3si6568083qao.74.2013.12.20.10.08.23
        for <clonemeagain@gmail.com>
        (version=TLSv1.1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);
        Fri, 20 Dec 2013 10:08:23 -0800 (PST)
Received-SPF: pass (google.com: domain of noreply@github.com designates 192.30.252.196 as permitted sender) client-ip=192.30.252.196;
Authentication-Results: mx.google.com;
       spf=pass (google.com: domain of noreply@github.com designates 192.30.252.196 as permitted sender) smtp.mail=noreply@github.com
Date: Fri, 20 Dec 2013 10:08:23 -0800
From: Jared Hancock <notifications@github.com>
Reply-To: "osTicket/osTicket-1.8" <reply+i-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH@reply.github.com>
To: "osTicket/osTicket-1.8" <osTicket-1.8@noreply.github.com>
Cc: clonemeagain <clonemeagain@gmail.com>
Message-ID: <osTicket/osTicket-1.8/pull/336/issue_event/82864993@github.com>
In-Reply-To: <osTicket/osTicket-1.8/pull/336@github.com>
References: <osTicket/osTicket-1.8/pull/336@github.com>
Subject: Re: [osTicket-1.8] Landing page inline image correction (#336)
Mime-Version: 1.0
Content-Type: multipart/alternative;
 boundary="--==_mimepart_52b4879729712_d621217cfc567e3";
 charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: list
X-GitHub-Recipient: clonemeagain
X-GitHub-Reason: author
List-ID: osTicket/osTicket-1.8 <osTicket-1.8.osTicket.github.com>
List-Archive: https://github.com/osTicket/osTicket-1.8
List-Post: <mailto:reply+i-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH@reply.github.com>
List-Unsubscribe: <mailto:unsub+i-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH@reply.github.com>,
 <https://github.com/notifications/unsubscribe/BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISH-BUNCHORANDOMGIBBERIBBERISHBUNCHORANDOMGIBBERIBBERISHBUNCHORANDOMGIBBERIBBERISH>
X-Auto-Response-Suppress: All
X-GitHub-Recipient-Address: clonemeagain@gmail.com
$setPriority
HEADER;
}
?>

There it is.. added total time counter.. ;-)

Laptop:
Greazys New: 0.135ms, total: 13.839s
Clonemeagain New: 0.027ms, total: 2.875s
MyVersion of Greazys: 0.072ms, total: 7.406s
Time to beat: 0.027ms

Crappy Debian:
Greazys New: 0.000ms, total: 28.049s
Clonemeagain New: 0.000ms, total: 17.540s
MyVersion of Greazys: 0.000ms, total: 20.576s
Time to beat: 0.027ms

Workstation:
Greazys New: 0.126ms, total: 12.822s
Clonemeagain New: 0.061ms, total: 6.373s
MyVersion of Greazys: 0.083ms, total: 8.541s
Time to beat: 0.027ms

Proxy:
Greazys New: 0.090ms, total: 9.162s
Clonemeagain New: 0.020ms, total: 2.144s
MyVersion of Greazys: 0.050ms, total: 5.108s
Time to beat: 0.027ms

My laptop is 5 years old.. I need to talk to my bosses about getting some proper servers.. FFFS.