[php] Check if a string contains numbers and letters

I want to detect if a string contains both numbers and letters.

For example:

• Given PncC1KECj4pPVW, it would be written to a text file because it contains both.
• Given qdEQ, it would not, because it only contains letters.

Is there a method to do this?

I was trying to use

$string = PREG_REPLACE("/[^0-9a-zA-Z]/i", '', $buffer);

But it didn't work.

Any help would be appreciated.