I am planning to write an application that I will make commercially available for others to download and install on their own servers for use.

While researching on the web, I read that Php 4 has some security issues ( sql injection etc.), and PHP 5 is OO and better, but my real concern is that others should be able to download and use it.

Which version of PHP is more appropriate - PHP 4, 4.1 or 5. I don't want to be in a situation that I write it as a OO application in PHP 5 and then it may not run for those people running PHP 4 on their shared hosting servers.

So my question is - how is the adoption of PHP 5. Do you all see most people using PHP 5 or still PHP 4. What's the trend and what is the direction.

I am planning for my application to be ready in 6 months time.

Any help is appreciated.