A class is an entity that encapsulates properties and methods. A class is made of properties (variables) and methods (functions), and can also contain class constants.
More can be found here: http://uk.php.net/class
Class constants are set within a class and can only be accessed by the class they were set in. They are created a little differently from a typical constant, but share the same characteristics: cannot be unset, changed, should be uppercase, underscore instead of a space. Class constants are defined like so:
const MY_CONSTANT = 'My Data';
Comments make your PHP script easier to read. You can create single line comments by placing // at the begining of the line, or multiline comments by starting the comment block with /* and ending it with */
When you compare two values or variables. Straight comparisons are done using two equal signs. For example - if ($myVar == $otherVar) - or - if ($myVar < $otherVar)
A condition is used to control loops and is generally a Boolean value
A constant is a way of storing a text item or number that cannot change. Constants cannot be unset or modified and are used for such things as database credentials. A typical constant may be created like so: