Руководство по PHP
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# Глава 16. Control Structures

Any PHP script is built out of a series of statements. A statement can be an assignment, a function call, a loop, a conditional statement or even a statement that does nothing (an empty statement). Statements usually end with a semicolon. In addition, statements can be grouped into a statement-group by encapsulating a group of statements with curly braces. A statement-group is a statement by itself as well. The various statement types are described in this chapter.

# `if`

The `if` construct is one of the most important features of many languages, PHP included. It allows for conditional execution of code fragments. PHP features an `if` structure that is similar to that of C:

    if (expr)
        statement

As described in the section about expressions, `expression` is evaluated to its Boolean value. If `expression` evaluates to `TRUE`, PHP will execute `statement`, and if it evaluates to `FALSE` - it'll ignore it. More information about what values evaluate to `FALSE` can be found in the 'Converting to boolean' section.

The following example would display `a is bigger than b` if `$a` is bigger than `$b`:

`<?php if ($a > $b) echo "a is bigger than b"; ?>`

Often you'd want to have more than one statement to be executed conditionally. Of course, there's no need to wrap each statement with an `if` clause. Instead, you can group several statements into a statement group. For example, this code would display `a is bigger than b` if `$a` is bigger than `$b`, and would then assign the value of `$a` into `$b`:

`<?php if ($a > $b) { echo "a is bigger than b"; $b = $a; } ?>`

`If` statements can be nested indefinitely within other `if` statements, which provides you with complete flexibility for conditional execution of the various parts of your program.

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Оператор проверки принадлежности к классу | Уровень выше | `else`