Difference between “include” and “require” in php

1.include("cart.inc") and require("cart.inc"), which should be used in PHP?

If cart.inc does not exist, include("cart.inc") will continue to execute with ignoring warning message,
while require("cart.inc") will halt the script.
It is better to use require("cart.inc") so we can know the error first.

2.Difference between echo, print, var_dump and print_r in PHP?
1) echo a string, not return value
2) print a string, return value 1
3) print_r, not only output a string, but also object type etc
4) var_dump, is similar to print_r, but produces more information than print_r.

3.What is difference between single quote and C in PHP?
The variable in double quote is evaluated, while in single quote it is left as it is.