Expressions
Expressions are the most important building stones of PHP. In PHP,
almost anything you write is an expression. The simplest yet
most accurate way to define an expression is "anything that has a
value".
The most basic forms of expressions are constants and variables.
When you type "$a = 5", you're assigning '5' into $a. '5', obviously,
has the value 5, or in other words '5' is an expression with the
value of 5 (in this case, '5' is an integer constant).
After this assignment, you'd expect $a's value to be 5 as
well, so if you wrote $b = $a, you'd expect it to behave just as
if you wrote $b = 5. In other words, $a is an expression with the
value of 5 as well. If everything works right, this is exactly
what will happen.
Slightly more complex examples for expressions are functions. For
instance, consider the following function:
Assuming you're familiar with the concept of functions (if you're
not, take a look at the chapter about functions), you'd assume
that typing $c = foo() is essentially just like
writing $c = 5, and you're right. Functions
are expressions with the value of their return value. Since foo()
returns 5, the value of the expression 'foo()' is 5. Usually
functions don't just return a static value but compute something.
Of course, values in PHP don't have to be integers, and very often
they aren't. PHP supports four scalar value types: integer
values, floating point values (float), string
values and boolean values (scalar values are values that you
can't 'break' into smaller pieces, unlike arrays, for instance). PHP also
supports two composite (non-scalar) types: arrays and objects. Each of
these value types can be assigned into variables or returned from functions.
PHP takes expressions much further, in the same way many other languages
do. PHP is an expression-oriented language, in the
sense that almost everything is an expression. Consider the
example we've already dealt with, '$a = 5'. It's easy to see that
there are two values involved here, the value of the integer
constant '5', and the value of $a which is being updated to 5 as
well. But the truth is that there's one additional value involved
here, and that's the value of the assignment itself. The
assignment itself evaluates to the assigned value, in this case 5.
In practice, it means that '$a = 5', regardless of what it does,
is an expression with the value 5. Thus, writing something like
'$b = ($a = 5)' is like writing '$a = 5; $b = 5;' (a semicolon
marks the end of a statement). Since assignments are parsed in a
right to left order, you can also write '$b = $a = 5'.
Another good example of expression orientation is pre- and
post-increment and decrement. Users of PHP and many other
languages may be familiar with the notation of variable++ and
variable--. These are
increment and decrement operators. In
PHP/FI 2, the statement '$a++' has no value (is not an
expression), and thus you can't assign it or use it in any way.
PHP enhances the increment/decrement capabilities by making
these expressions as well, like in C. In PHP, like in C, there
are two types of increment - pre-increment and post-increment.
Both pre-increment and post-increment essentially increment the
variable, and the effect on the variable is identical. The
difference is with the value of the increment expression.
Pre-increment, which is written '++$variable', evaluates to the
incremented value (PHP increments the variable before reading its
value, thus the name 'pre-increment'). Post-increment, which is
written '$variable++' evaluates to the original value of
$variable, before it was incremented (PHP increments the variable
after reading its value, thus the name 'post-increment').
A very common type of expressions are comparison
expressions. These expressions evaluate to either FALSE or TRUE. PHP
supports > (bigger than), >= (bigger than or equal to), == (equal),
!= (not equal), < (smaller than) and <= (smaller than or equal to).
The language also supports a set of strict equivalence operators: ===
(equal to and same type) and !== (not equal to or not same type).
These expressions are most commonly used inside conditional execution,
such as if statements.
The last example of expressions we'll deal with here is combined
operator-assignment expressions. You already know that if you
want to increment $a by 1, you can simply write '$a++' or '++$a'.
But what if you want to add more than one to it, for instance 3?
You could write '$a++' multiple times, but this is obviously not a
very efficient or comfortable way. A much more common practice is
to write '$a = $a + 3'. '$a + 3' evaluates to the value of $a
plus 3, and is assigned back into $a, which results in
incrementing $a by 3. In PHP, as in several other languages
like C, you can write this in a shorter way, which with time would
become clearer and quicker to understand as well. Adding 3 to the
current value of $a can be written '$a += 3'. This means exactly
"take the value of $a, add 3 to it, and assign it back into $a".
In addition to being shorter and clearer, this also results in
faster execution. The value of '$a += 3', like the value of a
regular assignment, is the assigned value. Notice that it is NOT
3, but the combined value of $a plus 3 (this is the value that's
assigned into $a). Any two-place operator can be used in this
operator-assignment mode, for example '$a -= 5' (subtract 5 from
the value of $a), '$b *= 7' (multiply the value of $b by 7), etc.
There is one more expression that may seem odd if you haven't seen
it in other languages, the ternary conditional operator:
If the value of the first subexpression is TRUE (non-zero), then
the second subexpression is evaluated, and that is the result of
the conditional expression. Otherwise, the third subexpression is
evaluated, and that is the value.
The following example should help you understand pre- and
post-increment and expressions in general a bit better:
Some expressions can be considered as statements. In
this case, a statement has the form of 'expr' ';' that is, an
expression followed by a semicolon. In '$b=$a=5;', $a=5 is a
valid expression, but it's not a statement by itself. '$b=$a=5;'
however is a valid statement.
One last thing worth mentioning is the truth value of expressions.
In many events, mainly in conditional execution and loops, you're
not interested in the specific value of the expression, but only
care about whether it means TRUE or FALSE.
The constants TRUE and FALSE (case-insensitive) are the two
possible boolean values. When necessary, an expression is
automatically converted to boolean. See the
section about
type-casting for details about how.
PHP provides a full and powerful implementation of expressions, and
documenting it entirely goes beyond the scope of this manual. The
above examples should give you a good idea about what expressions
are and how you can construct useful expressions. Throughout the
rest of this manual we'll write expr
to indicate any valid PHP expression.
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