PHP offers a lot of powerful operators, ranging from arithmetic, string, and logical operators to operators for assignment, comparison, and more (see Table 4-1).
Table 4-1. PHP operator types
Manipulating bits within bytes
Comparing two values
Executing contents of backticks
Adding or subtracting 1
Different types of operators take a different number of operands:
There is one ternary operator, which takes
x ? y : z. It’s a terse,
if statement that
chooses between two expressions, depending on the result of a third
one. This conditional operator takes three operands.
If all operators had the same precedence, they would be processed in the order in which they are encountered. In fact, many operators do have the same precedence—Example 4-5 illustrates one such case.
Here you will see that although the numbers (and their preceding
operators) have been moved around, the result of each expression is the
7, because the plus and minus
operators have the same precedence. We can try the same thing with
multiplication and division (see Example 4-6).
Example 4-6. Three expressions that are also equivalent
1 * 2 * 3 / 4 * 5 2 / 4 * 5 * 3 * 1 5 * 2 / 4 * 1 * 3
Here the resulting value is always
7.5. But things change when we mix operators
with different precedences in an expression, as in
Example 4-7. Three expressions using operators of mixed precedence
1 + 2 * 3 - 4 * 5 2 - 4 * 5 * 3 + 1 5 + 2 - 4 + 1 * 3
If there were no operator precedence, these three expressions
would evaluate to
12, respectively. But because multiplication
and division take precedence over addition and subtraction, there are
implied parentheses around these parts of the expressions, which would
look like Example 4-8 if
they were visible.
Example 4-8. Three expressions showing implied parentheses
1 + (2 * 3) - (4 * 5) 2 - (4 * 5 * 3) + 1 5 + 2 - 4 + (1 * 3)
Clearly, PHP must evaluate the subexpressions within parentheses first to derive the semi-completed expressions in Example 4-9.
Example 4-9. After evaluating the subexpressions in parentheses
1 + (6) - (20) 2 - (60) + 1 5 + 2 - 4 + (3)
The final results of these expressions are
6, respectively (quite different
from the results of
that we would have seen had there been no operator precedence).
Of course, you can override the default operator precedence by inserting your own parentheses and force the original results that we would have seen, had there been no operator precedence (see Example 4-10).
Example 4-10. Forcing left-to-right evaluation
((1 + 2) * 3 - 4) * 5 (2 - 4) * 5 * 3 + 1 (5 + 2 - 4 + 1) * 3
With parentheses correctly inserted, we now see the values
Table 4-2 lists PHP’s operators in order of precedence from high to low.
Table 4-2. The precedence of PHP operators (high to low)
Arithmetic and string
Bitwise (and references)
We’ve been looking at processing expressions from left to right, except where operator precedence is in effect. But some operators can also require processing from right to left. The direction of processing is called the operator’s associativity.
This associativity becomes important in cases in which you do not explicitly force precedence. Table 4-3 lists all the operators that have right-to-left associativity.
Table 4-3. Operators with right-to-left associativity
Create a new object
Increment and decrement
Unary plus and negation
Cast to an integer
Cast to a float
Cast to a string
Cast to an array
Cast to an object
Inhibit error reporting
For example, let’s take a look at the assignment operator in Example 4-11, where three variables are
all set to the value
This multiple assignment is possible only if the rightmost part of the expression is evaluated first and then processing continues in a right-to-left direction.
As a PHP beginner, you should learn to avoid the potential pitfalls of operator associativity by always nesting your subexpressions within parentheses to force the order of evaluation. This will also help other programmers who may have to maintain your code to understand what is happening.
The equality operator, which we’ve already encountered a few
times in this chapter, is
equals signs). It is important not to confuse it with the
= (single equals sign) assignment operator.
In Example 4-12, the
first statement assigns a value and the second tests it for
Example 4-12. Assigning a value and testing for equality
<?php $month = "March"; if ($month == "March") echo "It's springtime"; ?>
As you see, returning either
FALSE, the equality operator enables you to
test for conditions using, for example, an
if statement. But that’s not the whole
story, because PHP is a loosely typed language. If the two operands of
an equality expression are of different types, PHP will convert them
to whatever type makes best sense to it.
For example, any strings composed entirely of numbers will be
converted to numbers whenever compared with a number. In Example 4-13,
are two different strings and we would therefore expect neither of the
if statements to output a
Example 4-13. The equality and identity operators
<?php $a = "1000"; $b = "+1000"; if ($a == $b) echo "1"; if ($a === $b) echo "2"; ?>
However, if you run the example, you will see that it outputs
the number 1, which means that the first
if statement evaluated to
TRUE. This is because both strings were
first converted to numbers, and 1000 is the same numerical value as
In contrast, the second
statement uses the identity operator—three equals
signs in a row—which prevents PHP from automatically converting types.
$b are therefore compared as strings and are
now found to be different, so nothing is output.
In the same way that you can use the equality operator to test
for operands being equal, you can test for them
not being equal using
!=, the inequality
operator. Take a look at Example 4-14, which is a
rewrite of Example 4-13 in
which the equality and identity operators have been replaced with
Example 4-14. The inequality and not identical operators
<?php $a = "1000"; $b = "+1000"; if ($a != $b) echo "1"; if ($a !== $b) echo "2"; ?>
As you might expect, the first
if statement does not output the number 1,
because the code is asking whether
are not equal to each other numerically.
Instead, it outputs the number 2, because the second
if statement is asking whether
are not identical to each other in their present
operand types, and the answer is
TRUE; they are not the same.
Using comparison operators, you can test for more than just
equality and inequality. PHP also gives you
> (is greater than),
< (is less than),
>= (is greater than or equal to), and
<= (is less than or equal to) to
play with. Example 4-15 shows
these operators in use.
Example 4-15. The four comparison operators
<?php $a = 2; $b = 3; if ($a > $b) echo "$a is greater than $b<br />"; if ($a < $b) echo "$a is less than $b<br />"; if ($a >= $b) echo "$a is greater than or equal to $b<br />"; if ($a <= $b) echo "$a is less than or equal to $b<br />"; ?>
In this example, where
the following is output:
2 is less than 3 2 is less than or equal to 3
Try this example yourself, altering the values of
$b, to see the results. Try setting them to
the same value and see what happens.
Logical operators produce true-or-false results, and therefore are also known as Boolean operators. There are four of them (see Table 4-4).
Table 4-4. The logical operators
You can see these operators used in Example 4-16. Note that the
! symbol is required by PHP in place of the
NOT. Furthermore, the
operators can be lower- or uppercase.
Example 4-16. The logical operators in use
<?php $a = 1; $b = 0; echo ($a AND $b) . "<br />"; echo ($a or $b) . "<br />"; echo ($a XOR $b) . "<br />"; echo !$a . "<br />"; ?>
This example outputs
meaning that only the second and third
echo statements evaluate as
TRUE. (Remember that
NULL—or nothing—represents a value of
FALSE.) This is because the
AND statement requires both
operands to be
TRUE if it is going
to return a value of
the fourth statement performs a
on the value of
$a, turning it from
TRUE (a value of
FALSE. If you wish to experiment with this,
try out the code, giving
$b varying values of
When coding, remember to bear in mind that
OR have lower precedence than the other
versions of the operators,
||. In complex expressions, it may be
safer to use
|| for this reason.
OR operator can cause
unintentional problems in
statements, because the second operand will not be evaluated if the
first is evaluated as
Example 4-17, the function
getnext will never be called if
$finished has a value of
If you need
getnext to be
called at each
if statement, you
could rewrite the code as has been done in Example 4-18.
Example 4-18. The if…OR statement modified to ensure calling of getnext
<?php $gn = getnext(); if ($finished == 1 OR $gn == 1) exit; ?>
In this case, the code in function
getnext will be executed and the value
returned stored in
$gn before the
Another solution is to simply switch the two clauses to make
getnext is executed, as
it will then appear first in the expression.
Table 4-5 shows
all the possible variations of using the logical operators. You should
also note that