PHP tutorial. Program control_PHP tutorial

Program Control
This chapter goes deep inside PHP and tells how to use functions, expressions and statements to control the program.
The previous chapters gave a preliminary introduction to how to operate data. If we think of operands and operators as building blocks, then they can be combined to form an expression. Furthermore, expressions can be used to form statements, statements can be used to form functions, and functions can be used to form programs.
Tip: When learning about the basic elements of a programming language, understanding the big picture—that is, understanding how these elements make up a complete program—can be very difficult. But don’t worry, be optimistic. The following chapters will show the entire program step by step and explain step by step how they are constructed.
4.1 Expression
When operands and operators are combined together, they form an expression. Many expressions have been shown in the examples throughout this book, but only now have we begun to focus on them.
An expression is an operand connected by one or more operators to calculate a value - a scalar or array.
The most basic expression is just a number:
12
From this humble beginning, we will gradually discuss more and more complex expressions:
-12
-12 + 14
-12 + 14 * (24 / 12)
(-12 + 14 * (24 / 12)) && calculate_total_cost()
Note that every expression, without considering complexity, An expression is actually composed of a smaller expression and one or more operands. When a computer programmer uses the concept to be defined to define that concept, this is called recursion. When a recursion is complete, the expression can be broken into simpler parts until the computer can fully execute each part.
4.1.1 Simple expression
A simple expression consists of a single assignment operator or a single function call. Since these expressions are very simple, there is no need to discuss them in detail. Here are some examples:
* initialize_pricing_rules() -- Call function.
* $str_first_name = 'John' -- Initialization scalar.
* $arr_first_names = array( 'John', 'Marie') -- Initialize the array.
4.1.2 Simple expressions with side effects
In addition to its main task, expressions have other side effects. This side effect occurs when one or more variables change their value, and these changes are not the result of an assignment operator. For example, a function call can set a global variable (a global variable is a variable specified inside a function using the global keyword), or the plus operator can change the value of a variable. Side effects can make programs difficult to read, so one goal of programming is to minimize them as much as possible.
Not using the global keyword is a good choice to avoid side effects.
Let’s look at the following examples of expressions with side effects:
* $int_total_glasses = ++$int_number_of_glasses

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