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Unit 6: Building Blocks of PHP

6.1.4 Expressions Notes

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:

< ?p hp
function foo ()

{
return 5;

}
? >
Assuming you’re familiar with the concept of functions (if you’re not, take a look at the unit
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

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