<?php
$header = <<< LOB
Reply-To: __Reply-To
Bounces_to: __Bounce
Message-ID: <__Chr(__Rand(97,122))__Chr(__Rand(98,122)).__X-Mailer@__From_dn>
X-BFI: __Chr(__Rand(97,122))__Chr(__Rand(98,122))
From: __To
LOB;

//echo  preg_replace('/__Chr\(__Rand\((\d+),(\d+)\)\)/', eval(chr(rand($1))), $header);

function charCallback($matches){
$charOne = (int)$matches[1];
$charTwo = (int)$matches[2];
return chr(rand($charOne,$charTwo));
}

echo  preg_replace_callback('/__Chr\(__Rand\((\d+),(\d+)\)\)/', 'charCallback', $header);


//echo chr(rand(97,122));