So I was reading about PHP namespaces, and I realized that in versions earlier than 5.3, if you write
namespace MyNamespace
you get a parse error.
Is there any way to avoid this i.e. make namespaces backwards-compatible, so the code doesn't simply crash?
Long Answer: No.
Longer Answer: (added to capture useful information from other deleted answers). The new Syntax will cause parse errors in PHP, so you can't use a customer error handler to catch errors generated in versions < 5.3. In theory you could write a pre-processor the scans and/or does a lex/parse on the source and then write something back out that would be PHP 5.2 compatible, but that creates more problems than it solves.
Perhaps you could query the version of PHP being used and call eval if it's high enough. I don't know if that will work though.
Actually, I think it's possible, but I don't believe it's worth it. The idea would be to create a custom default stream wrapper, which will parse PHP files according to the new grammar and make the appropriate changes to the syntax so that it will be valid PHP < 5.3.
The wrapper would have to replace class names such as Foo\Bar\Baz with Foo_Bar_Baz. Currently I'm not sure if there's something that would render this impossible.
Anyway, I don't believe it's worth the effort. Upgrade to PHP 5.3.
Oh, that means that the wrapper code should be compatible with PHP < 5.3.