在你回答这个问题之前，我从来没有开发过任何流行到足以达到高服务器负载的东西。请把我当作(唉)一个刚刚登陆地球的外星人，尽管我知道PHP和一些优化技术。
我正在开发一个PHP工具，可以获得相当多的用户，如果它是正确的。然而，虽然我完全有能力开发程序，但当涉及到制作可以处理巨大流量的东西时，我几乎一无所知。所以这里有一些关于它的问题(也可以把这个问题变成一个资源线程)。
数据库
At the moment I plan to use the MySQLi features in PHP5. However how should I setup the databases in relation to users and content? Do I actually need multiple databases? At the moment everything's jumbled into one database - although I've been considering spreading user data to one, actual content to another and finally core site content (template masters etc.) to another. My reasoning behind this is that sending queries to different databases will ease up the load on them as one database = 3 load sources. Also would this still be effective if they were all on the same server?
缓存
我有一个用于构建页面和交换变量的模板系统。主模板存储在数据库中，每当一个模板被调用时，它的缓存副本(html文档)就会被调用。目前，我在这些模板中有两种类型的变量-静态变量和动态变量。静态变量通常是像页面名称，网站的名称-不经常改变的东西;动态变量是在每次页面加载时改变的东西。
我的问题是:
比如说我对不同的文章有评论。这是一个更好的解决方案:存储简单的注释模板，并在每次页面加载时呈现注释(来自DB调用)，或者将注释页面的缓存副本存储为html页面——每次添加/编辑/删除注释时，页面都会被重新检索。
最后
有人有任何提示/指针运行一个高负载的PHP网站。我很确定这是一种可行的语言——Facebook和Yahoo!优先考虑——但有什么经验是我应该注意的吗?
一般
在开始看到真实世界的负载之前，不要尝试优化。你可能猜对了，但如果你猜错了，那你就是在浪费时间。
使用jmeter、xdebug或其他工具对站点进行基准测试。
如果加载开始成为一个问题，对象或数据缓存都可能涉及到，所以通常阅读缓存选项(memcached, MySQL缓存选项)
Code
对代码进行分析，以便了解瓶颈在哪里，以及它是在代码中还是在数据库中
数据库
Use MYSQLi if portability to other databases is not vital, PDO otherwise
If benchmarks reveal the database is the issue, check the queries before you start caching. Use EXPLAIN to see where your queries are slowing down.
After the queries are optimized and the database is cached in some way, you may want to use multiple databases. Either replicating to multiple servers or sharding (splitting the data over multiple databases/servers) may be appropriate, depending on the data, the queries, and the kind of read/write behavior.
缓存
Plenty of writing has been done on caching code, objects, and data. Look up articles on APC, Zend Optimizer, memcached, QuickCache, JPCache. Do some of this before you really need to, and you'll be less concerned about starting off unoptimized.
APC and Zend Optimizer are opcode caches, they speed up PHP code by avoiding reparsing and recompilation of code. Generally simple to install, worth doing early.
Memcached is a generic cache, that you can use to cache queries, PHP functions or objects, or entire pages. Code must be specifically written to use it, which can be an involved process if there are no central points to handle creation, update and deletion of cached objects.
QuickCache and JPCache are file caches, otherwise similar to Memcached. The basic concept is simple, but also requires code and is easier with central points of creation, update and deletion.
杂项
考虑高负载的替代web服务器。像lighthttp和nginx这样的服务器可以用比Apache少得多的内存处理大量流量，如果你可以牺牲Apache的强大功能和灵活性(或者如果你不需要这些东西，通常情况下，你不需要)。
请记住，现在的硬件非常便宜，所以一定要花费精力来优化一大块代码，而不是“让我们购买一个巨型服务器”。
考虑将“MySQL”和“scaling”标签添加到这个问题中
