' (Single Quotes) in user inputs

I am running into a problem with user input when a user inputs a single quote in the form field, such as entering O'Brien as a last name. When the user enters this, PHP adds a \ in front of the single quote, and then if the form is submitted again, adds a \ in front of the \ and again in front of the single quote. Example: <INPUT TYPE="TEXT" NAME="lastName" VALUE="$lastName"> if the user enters a name containing a single quote, such as O'Brien, it becomes <INPUT TYPE="TEXT" NAME="lastName" VALUE="O\'Brien"> Upon submitting the form again, it become <INPUT TYPE="TEXT" NAME="lastName" VALUE="O\\\'Brien"> Any ideas on dealing with this would be greatly appreciated. Thanks. Ben