How many files in a folder?

I would like to know, up to how many files would it be safe to put in a folder and keep it working good? Would something like 1500 files (all .php) in the same folder be ok?

i think yes. it does not matter if they are in a folder if they are on the same partition since they share the same physical location.

maybe you want to install another filesystem than ext2 like reiserfs or xfs. they support storing filenames in binary trees which makes them find files faster.

for maintenance, i think you should not have more than 20-30 files in one folder. why don´t you use sub-folders?

To build on what M.Hirsch says, there's probably no real "problem" with lumping 1500 files into one directory, but finding anything in particular can become torturous. I have gazillions of subdirectories in my document root (throughout my filesystem, really), and I think it really helps with organization. For example, I have a subdirectory for a new site I'm designing, another subdirectory for a gallery of photos I've taken, etc. It makes things *much* more convenient when you have to find something.