Using Operators and Expressions in PHP

Forming Expressions Using Operators in PHP

Expressions are elements that can be evaluated to produce some value during the program execution. The simplest expressions are literals and variables. 

A literal evaluates to itself, while a variable evaluates to the value stored in the variable.More complex expressions can be formed using simple expressions and operators. An operator is a symbol used to specify the operation to be performed on operands. Some operators modify their operands, while most do not.

PHP borrows most of its operands from C and Perl. Most operators in PHP are binary operators; they combine two operands (or expressions) into a single, more complex expression. PHP also supports a number of unary operators and a single ternary operator that combines three expressions into a single expression.

Operator Precedence & Associativity:

The order in which operators in an expression are evaluated depends on their relative precedence. For e.g., among arithmetic operators, the operators * and / are in the higher precedence (evaluated first). And the operators + and – are having lower precedence (evaluated last).

To force a particular order, we can group operands with the appropriate operators in parenthesis. Operators placed within parenthesis are evaluated first (i.e., they get the highest priority to be executed first). But, within parenthesis, they are evaluated in the order of their precedence. For example, the expression:

(2 + 4 * 3) / 2

evaluates the expression to 7 – multiplication first, addition next (with in parenthesis) and then divide the result by 2. This is because of the highest priority given to the expression within parenthesis. The remaining operators, which are placed outside the parenthesis, are evaluated in their order of priority.

Associativity defines the order in which operators with the same order of precedence are evaluated. For example, the expression (2 / 2 * 2) is evaluated as:

(2 / 2) * 2 = 2

and not as: 2 / (2 * 2) = 0.5

Here, the associativity is left to right; that means, in case of ambiguity (expression having operators with same precedence), the operators are evaluated from left to right.

Implicit Casting:

Many operators in PHP require that both their operands are of same type. PHP variables can store values of any data type, which include integers, floating-point numbers, strings and more. The conversion of a value from one type to another is called casting.

PHP converts values from one type to another as necessary. This kind of casting (automatic conversion) is called implicit casting or type juggling in PHP. The rules for type juggling done by arithmetic operators are shown in the below table:

Type of First Operand	Types of Second Operand	Conversion Performed
Integer	Floating-point number	The integer is converted to a floating-point number
Integer	String	The string is converted to a number; if the value after conversion is a floating-point number, the integer is converted to a floating-point number.
Floating-point number	String	The string is converted to a floating-point number

Table: Implicit Casting rules for Binary Arithmetic Operators

We can use a string anywhere PHP expects a number. The string is presumed to start with an integer or floating-point number. If no number is found at the start of the string, the numeric value of that string is 0. If the string contains a period (.) or upper- or lower-case e, evaluating it numerically produces a floating-point number. For e.g.,

“9 Lives” -1; //8 (int)

“3.14 Pies” * 2; //6.28 (float)
             

“1E3 Points of Light” + 1 //1001 (float)

Types of Operators:

Based on their usage, operators are of various types:

1. Arithmetic operators
2. String Concatenation operator
3. Auto increment and Auto Decrement operator
4. Comparison operators
5. Bitwise operators
6. Logical operators
7. Casting operators
8. Assignment operators
9. Miscellaneous operators

The first and foremost operator of all PHP operators is arithmetic operators. Arithmetic operators are set of operators that perform arithmetic operations on numeric values. Most of the arithmetic operators are binary; however, the arithmetic negation (-) and arithmetic assertion (+) operators are unary.

Binary arithmetic operators are: +, -, *, / and % and they perform operations like addition, subtraction, multiplication, division and modulo division respectively. If they are applied on non-numeric values, they are converted into numeric values automatically.

The unary arithmetic operator arithmetic negation (-) multiplies the value of its operand by -1 and hence changes its sign. For e.g., -(3-4) evaluates to 1. The arithmetic assertion operator (+) returns the operand multiplied by +1, which has no effect. It is used as a visual cue to indicate true sign of a value.

The second type of operator in PHP is String Concatenation operator. This operator is denoted by the symbol dot (.), which appends the right-hand operand to the left-hand operand and returns the resulting string. Operands are first converted to strings, if necessary. For e.g.:

$n = 5;

$s = ‘5 +’ . $n . ‘is 10’; //$s is ‘5 + 5 is 10’

Here is a Presentation to have an Overview of PHP