Regular expression to remove anything but alphanumeric & spaces (in PHP)

I'm trying to remove, via regular expression, all but alphanumeric characters & spaces.

Here's the conversion I am hoping to achieve.

"I am a string" → "I am a string"
"How are you?" → "How are you"
"#53-Jeff" → "53-Jeff"

So far I have this:

return preg_replace("/[^0-9a-zA-Z]/","", $val);

But being a regex novice, I can't figure out how to insert a space. I had odd results when I tried.