[logn]

A lot of engineers will gladly work overtime or at home to pick up a skill. Often you don't start a new job for a few weeks or month.

And any good engineer can work blind on a new technology and still produce decent results. If a great engineer with no knowledge of PHP is required to build a PHP website they can sit down with a PHP book open on their desk and crank it out. They fact that they're mentally superior to other engineers (whose expertise on a narrow subject might be better) and know programming inherently better means there's a good chance their output on something they don't know could be comparable to the PHP coder.

The best managers always build skills in teams. It's part of compensation. Every engineer views bullet points on resumes as a form of income. "If they don't know PHP, they're not going to be learning it on my time" means that you're going to be paying higher salaries with higher turnover and much higher recruiting costs. And if that's not true then something scarier is true: you're actually hiring low-quality engineers who don't realize you're a bad manager with a bad team (or who have no other options than to pimp out their 8 years of SharePoint 2003 experience).

Maybe you wouldn't have such an urgent need to hire experts in narrow subjects if you had staffed your team with intelligent quick learners in the first place! Tip: next time you need an expert in subject X try telling your best engineer: hey I have a project coming up for an X project and I'd like you to play a key part, so can you study up on that as it'd really impress me.