operator

An operator is a symbol or series of symbols that, when used in conjunction with values, performs an action and usually produces a new value.

operand

An operand is a value used in conjunction with an operator. There are usually two operands to one operator.

Expression

Expression is any combination of functions, values, and operators that resolves to a value. As a rule of thumb, if you can use it as if it were a value, it is an expression.

Constants

Variables offer a flexible way of storing data because you can change their values and the type of data they store at any time. If, however, you want to work with a value that you do not want to alter throughout your script's execution, you can define a constant. You must use PHP's built-in function define() to create a constant. After you have done this, the constant cannot be changed. To use the define() function, you must place the name of the constant and the value you want to give it within the call's parentheses. These values must be separated by a comma, like so:

define ("CONSTANT_NAME", 42);

The if Statement

An if statement is a way of controlling the execution of a statement that follows it (that is, a single statement or a block of code inside braces). The if statement evaluates an expression between parentheses. If this expression results in a true value, the statement is executed. Otherwise, the statement is skipped entirely. This enables scripts to make decisions based on any number of factors.

What Is a Function?

A function, then, is a self-contained block of code that can be called by your scripts. When called, the function's code is executed. You can pass values to functions, which they then work with. When finished, a function can pass a value back to the calling code.

What Is an Array?

An array enables you to store as many values as you want in the same variable.

Type Specifies

Specifier	Description
d	Displays an argument as a decimal number
b	Displays an integer as a binary number
c	Displays an integer as its ASCII equivalent
f	Displays an integer as a floating-point number (double)
o	Displays an integer as an octal number (base 8)
s	Displays an argument as a string
x	Display an integer as a lowercase hexadecimal number (base 16)
X	Displays an integer as an uppercase hexadecimal number (base 16)