WordPress/wp-includes/vars.php

<?php

// On which page are we ?
if ( preg_match('#([^/]+\.php)$#', $PHP_SELF, $self_matches) ) {
	$pagenow = $self_matches[1];
} elseif ( strpos($PHP_SELF, '?') !== false ) {
	$pagenow = explode('/', $PHP_SELF);
	$pagenow = trim($pagenow[(sizeof($pagenow)-1)]);
	$pagenow = explode('?', $pagenow);
	$pagenow = $pagenow[0];
} else {
	$pagenow = 'index.php';
}

// Simple browser detection
$is_lynx = $is_gecko = $is_winIE = $is_macIE = $is_opera = $is_NS4 = false;

if (strpos($_SERVER['HTTP_USER_AGENT'], 'Lynx') !== false) {
	$is_lynx = true;
} elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'Gecko') !== false) {
	$is_gecko = true;
} elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false && strpos($_SERVER['HTTP_USER_AGENT'], 'Win') !== false) {
	$is_winIE = true;
} elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE') !== false && strpos($_SERVER['HTTP_USER_AGENT'], 'Mac') !== false) {
	$is_macIE = true;
} elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'Opera') !== false) {
	$is_opera = true;
} elseif (strpos($_SERVER['HTTP_USER_AGENT'], 'Nav') !== false && strpos($_SERVER['HTTP_USER_AGENT'], 'Mozilla/4.') !== false) {
	$is_NS4 = true;
}

$is_IE = ( $is_macIE || $is_winIE );

// Server detection
$is_apache = ((strpos($_SERVER['SERVER_SOFTWARE'], 'Apache') !== false) || (strpos($_SERVER['SERVER_SOFTWARE'], 'LiteSpeed') !== false)) ? true : false;
$is_IIS = (strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS') !== false) ? true : false;

?>