Một biến luôn luôn bắt đầu với một ký hiệu đô la, $, mà sau đó được theo sau bởi tên biến. Tên biến theo các quy tắc đặt tên tương tự như nhận dạng. Đó là, một tên biến có thể bắt đầu với một lá thư hoặc một gạch dưới và có thể bao gồm các chữ cái, gạch dưới, con số, hoặc các ký tự ASCII khác nhau, từ 127 đến 255. | 74 CHAPTER 3 PHP BASICS Given the importance of this programming concept it would be wise to explicitly lay the groundwork as to how variables are declared and manipulated. In this section these rules are examined in detail. Note A variable is a named memory location that contains data and may be manipulated throughout the execution of the program. Variable Declaration A variable always begins with a dollar sign which is then followed by the variable name. Variable names follow the same naming rules as identifiers. That is a variable name can begin with either a letter or an underscore and can consist of letters underscores numbers or other ASCII characters ranging from 127 through 255. The following are all valid variables color operating_system _some_variable model Note that variables are case sensitive. For instance the following variables bear absolutely no relation to one another color Color COLOR Interestingly variables do not have to be explicitly declared in PHP as they do in Perl. Rather variables can be declared and assigned values simultaneously. Nonetheless just because you can do something doesn t mean you should. Good programming practice dictates that all variables should be declared prior to use preferably with an accompanying comment. CHAPTER 3 PHP BASICS 75 Once you ve declared your variables you can begin assigning values to them. Two methodologies are available for variable assignment by value and by reference. Both are introduced next. Value Assignment Assignment by value simply involves copying the value of the assigned expression to the variable assignee. This is the most common type of assignment. A few examples follow color red number 12 age 12 sum 12 15 sum 27 Keep in mind that each of these variables possesses a copy of the expression assigned to it. For example number and age each possesses their own unique copy of the value 12. If you prefer that two variables point to the same copy of a value you need to assign by reference introduced
