Checking a string for a certain character with PHP.?

[Nik]

What I want here is kind of like an anti-spam check

so I want to check a string for the amount of @ within it.

So if PHP finds 1 @ it does nothing and if it finds 2 or more it reports an error.

so

[email protected], [email protected]

the above would return an error but

[email protected]

would be ok

How would I do this.
Thanks