+ (Addition): 15 + 4 = 19
- (Subtraction): 15 - 4 = 11
* (Multiplication): 15 * 4 = 60
/ (Division): 15 / 4 = 3.75
++ (Increment): 5 incremented = 6
-- (Decrement): 5 decremented = 4
Addition combines numbers, subtraction finds the difference, multiplication repeats addition, and division splits into equal parts. Increment increases a value by 1, and decrement decreases a value by 1. These operators are commonly used in PHP to update counters and values in loops or user input.