php, escape quotes in html input value

In form
 <input type="text" name="firstname" value="<?php echo $_POST['firstname']; ?>" />
You may find if  

$_POST['firstname']=O'test may not work.
We need to use  htmlspecialchars or  htmlentities()

But we need to use  ENT_QUOTES to esacpe single, i.e.
 htmlspecialchars($_POST['firstname'],ENT_QUOTES) 

More about htmlspecialchars:

• '&' (ampersand) becomes '&'
• '"' (double quote) becomes '"' when ENT_NOQUOTES is not set.
• "'" (single quote) becomes ''' (or ') only when ENT_QUOTES is set.
• '<' (less than) becomes '&lt;'
• '>' (greater than) becomes '&gt;'