I've heard mixed things about this. I know it's easy when you're stuck to simply enable PHP in the template and hack your way around a problem. However, it almost always introduces potential security issues, and makes a mess of your template to boot.
In most cases where PHP is used, it would be cleaner to write a simple plugin in PHP to achieve what you need.
My question is this: Is it ever acceptable to use PHP inside ExpressionEngine templates? Or is this considered bad practice? If you were a developer who inherited a site I coded, would you cringe if you saw PHP inside templates?
This could just as easily be a question on the merits of eval() and whether it should ever be used. Most of the answers to questions along those lines are equally applicable here.
My view is that enabling PHP in templates is always a Bad Idea. At best, it's not a terrible idea, but there's always a better option. A custom addon will invariably be a better approach, even with seemingly harmless code snippets. I think it's also most likely to be used by those least qualified to use it. Personally, when I started building sites with EE my PHP skills ranged somewhere between 'totally inept' and 'knows enough to be dangerous'. At that time, if I came across a limitation in EE's code I'd often implement a solution via some inline PHP, partly because the idea of developing a custom addon seemed too daunting and partly because clicking a little 'enable PHP' radio button was so simple. Years later, whenever I need to update one of these sites, I definitely do cringe. Here are some of the side effects of PHP in templates:
With tools like Pkg.io you can have the groundwork for an addon in place in 30 seconds. It requires a tiny amount of extra effort, but the gains are well worth it.