$str = "WEB means a server system using world wide web technology."; //検索される文字列
$str1= "means"; //キーワードその１
$str2= "using"; //キーワードその２

// 正規表現あり
$delim = '/';
$pattern = sprintf("%s(?:%s)(.*?)(?:%s)%s", $delim, preg_quote($str1, $delim), preg_quote($str2, $delim), $delim);
preg_match($pattern, $str, $arr_str );
$match_str = $arr_str[1];
print $match_str . "\n";

// 正規表現なし
$match_str = '';
if (($str_work = strstr($str, $str1)) !== false) {
$str_work = substr($str_work, strlen($str1));
if (($i = strpos($str_work, $str2)) !== false) {
$match_str = substr($str_work, 0, $i);
}
}
print $match_str . "\n";