php expression

Expressions are the most important cornerstone of PHP. In PHP, almost everything you write is an expression. The simple but most precise way to define an expression is "anything with a value".

The most basic expression forms are constants and variables. When you type "$a = 5", the value "5" is assigned to the variable $a. "5", obviously, has the value 5, in other words "5" is an expression with the value 5 (here, "5" is an integer constant).

After the assignment, the expectation is that $a has a value of 5, so if we write $b = $a, the expectation is that it will be the same as $b = 5. In other words, $a is also an expression with the value 5. If everything works correctly, this is exactly what is going to happen.

An example of a slightly more complex expression is a function. For example, consider the following function:

function foo ()
{
return 5;
}
?>

Assuming you are already familiar with the concept of functions, then typing $c = foo() From the essence Basically it's like writing $c = 5, which is correct. Functions are also expressions, and the value of an expression is their return value. Since foo() returns 5, the expression "foo()" also evaluates to 5. Often functions don't just return a static value, but may compute something.

Of course, values ​​in PHP are often not integers. PHP supports four types of scalar values ​​(scalar values ​​cannot be split into smaller units, such as arrays): integer, float, string and boolean (boolean). PHP also supports two composite types: arrays and objects. Both types can be assigned to variables or returned from functions.

PHP develops along the expression path like other languages, but goes further. PHP is an expression-oriented language, in the sense that almost everything is an expression. Consider the example we just studied, "$a = 5". Obviously there are two values ​​involved here, the value of the integer constant "5" and the value of the variable $a, which is also updated to 5. But the fact is that there is an additional value involved, namely the value of the value-added statement itself. The assignment statement itself evaluates to the assigned value, which is 5. Effectively this means that "$a = 5", regardless of what it does, is an expression that evaluates to 5. Thus, writing "$b = ($a = 5)" is the same as writing "$a = 5; $b = 5" (the semicolon marks the end of the statement). Because the order of assignment operations is from right to left, you can also write "$b = $a = 5".

Another good expression-oriented example is front and back increment and decrement. Users of PHP and most other languages ​​should be familiar with the variable++ and variable-- symbols. Namely the increment and decrement operators. In PHP, like the C language, there are two types of increment - pre-increment and post-increment. Essentially, both pre-increment and post-increment increase the value of the variable, and the impact on the variable is the same and different. is the value of the incrementing expression. Pre-increment, written as "++$variable", finds the value after increment (PHP increments the value of the variable before reading the value of the variable, so it is called "pre-increment"). Post-increment, written as "$variable++", finds the original value of the variable before it is incremented (PHP increments the value of the variable after reading the value of the variable, so it is called "post-increment").

A commonly used expression type is a comparison expression. These expressions evaluate to FALSE or TRUE. PHP supports > (greater than), >= (greater than or equal to), == (equal to), != (not equal to), < (less than), and <= (less than or equal to). PHP also supports the equality operator === (same values ​​and types) and the non-identity operator !== (different values ​​or types). These expressions are often used in conditional judgment statements, such as if statements.

Here, the last example that we will study is the combined arithmetic assignment expression. Already know that if you want to add 1 to the variable $a, you can simply write "$a++" or "++$a". But what if you want to add a value greater than 1 to a variable, say 3? You can write "$a++" multiple times, but this is obviously not an efficient and comfortable method. A more general approach is "$a = $a + 3". "$a + 3" is equivalent to $a plus the value of 3, and the resulting value is reassigned to the variable $a, so the value of $a is increased by 3. In PHP and several other languages ​​such as C, the above functionality can be accomplished in a shorter form, and therefore more clearly and quickly. To add 3 to the current value of $a, you can write: "$a += 3". What it means here is "take the value of variable $a , add 3, and assign the result to variable $a again." In addition to being simpler and clearer, it can also run faster. The value of "$a += 3", like the value of a normal assignment operation, is the value after assignment. Note that it is not 3, but the value of $a plus 3 (this value will be assigned to $a). Any binary operator can use the operation assignment pattern, such as "$a -= 5" (subtract 5 from the value of variable $a), "$b *= 7" (multiply variable $b by 7), etc. wait.

There is also an expression that may look strange if you haven’t seen it in other languages, the ternary conditional operator:

<?php
$first ? $second : $third
?>

If the first subexpression evaluates to TRUE (non-zero) ), then the second subexpression is evaluated, and its value is the value of the entire conditional expression. Otherwise, the third subexpression is evaluated and its value becomes the value of the entire expression.

The following examples should help understand pre- and post-increment and expressions in general:

<?php
function double($i)
{
return $i*2;
}
$b = $a = 5;        /* assign the value five into the variable $a and $b */
$c = $a++;          /* post-increment, assign original value of $a
(5) to $c */
$e = $d = ++$b;     /* pre-increment, assign the incremented value of
$b (6) to $d and $e */
/* at this point, both $d and $e are equal to 6 */
$f = double($d++);  /* assign twice the value of $d before
the increment, 2*6 = 12 to $f */
$g = double(++$e);  /* assign twice the value of $e after
the increment, 2*7 = 14 to $g */
$h = $g += 10;      /* first, $g is incremented by 10 and ends with the
value of 24. the value of the assignment (24) is
then assigned into $h, and $h ends with the value
of 24 as well. */
?>

Some expressions can be treated as statements. At this time, the form of a statement is expr;, that is, an expression ends with a semicolon. In '$b = $a = 5;' , '$a = 5' is a valid expression, but it is not a statement in itself. And '$b = $a = 5;' is a valid statement.

The last thing worth mentioning is the truth value of expressions. In many events, mostly in conditional execution and loops, you don't care about the specific value in the expression, but only whether the value of the expression is TRUE or FALSE. The constants TRUE and FALSE (case-independent) are two possible Boolean values. An expression is automatically converted to a Boolean value when necessary.