PHP - 用preg_replace替换Tab空格？

I have a form which currently accepts a tabbled white space " " for example. I only want to allow one space between words.

Currently it looks like this:

preg_replace ("#[^0-9a-z]#i", " ",$string);

Remember I need to allow only one space at a time, How do I fix this?

Replace multiple whitespaces with a single space

$string = preg_replace('/\s\s+/',' ', $string);