[PHP] Bad characters in a text field

[Manuel Ochoa]

I have a form with a "text area" that when submited, get stored in a "text" type field 
of a MySQL database. Occasionaly some of our users copy data from a DOS program and 
paste it in the text area which results in unwanted characters getting stored in the 
text field. What function can I use to strip out all but valid characters?
 
For example: I want to allow a-z ,A-Z, 0-9 and some others and strip out everything 
else.