PHP

Expressions and Operators

In PHP there are two kinds of "expressions" you will hear about. One is the regular expression, used to manipulate text (Chapter 12). The other is more mathematical – it describes operations to be performed on values and variables:

$c = $a + $b;

Assuming $a and $b are numbers, this expression adds them and stores the sum in $c (the = sign makes it an assignment). In $a + $b the plus sign is an arithmetic operator; $a and $b are its operands.

Arithmetic Operators

<?php
$a = 17;
$b = 5;
echo "a + b = " . ($a + $b) . "<br>";
echo "a - b = " . ($a - $b) . "<br>";
echo "a * b = " . ($a * $b) . "<br>";
echo "a / b = " . ($a / $b) . "<br>";
echo "a % b = " . ($a % $b) . "<br>";
echo "a ** 2 = " . ($a ** 2) . "<br>";
echo "intdiv(a, b) = " . intdiv($a, $b);
?>

a + b = 22
a - b = 12
a * b = 85
a / b = 3.4
a % b = 2
a ** 2 = 289
intdiv(a, b) = 3

Comparison Operators

A comparison returns a boolean – true or false.

<?php
$x = 5;       // integer
$y = "5";     // string
echo "<pre>";
echo '$x == $y  : '; var_dump($x == $y);
echo '$x === $y : '; var_dump($x === $y);
echo '$x != 8   : '; var_dump($x != 8);
echo '$x !== $y : '; var_dump($x !== $y);
echo '$x >= 5   : '; var_dump($x >= 5);
echo '3 <=> 7   : '; var_dump(3 <=> 7);
echo '7 <=> 7   : '; var_dump(7 <=> 7);
echo "</pre>";
?>

$x == $y  : bool(true)
$x === $y : bool(false)
$x != 8   : bool(true)
$x !== $y : bool(true)
$x >= 5   : bool(true)
3 <=> 7   : int(-1)
7 <=> 7   : int(0)

Logical Operators

<?php
$age = 20;
$hasId = true;
echo "<pre>";
echo "age >= 18 && hasId : "; var_dump($age >= 18 && $hasId);
echo "age < 18 || hasId  : "; var_dump($age < 18 || $hasId);
echo "true xor true      : "; var_dump(true xor true);
echo "!hasId             : "; var_dump(!$hasId);
echo "</pre>";
?>

age >= 18 && hasId : bool(true)
age < 18 || hasId  : bool(true)
true xor true      : bool(false)
!hasId             : bool(false)

Increment and Decrement Operators

<?php
$v = 5;
echo "v++ gives " . $v++ . ", now v = $v<br>";
echo "++v gives " . ++$v . ", now v = $v<br>";
echo "v-- gives " . $v-- . ", now v = $v<br>";
echo "--v gives " . --$v . ", now v = $v";
?>

v++ gives 5, now v = 6
++v gives 7, now v = 7
v-- gives 7, now v = 6
--v gives 5, now v = 5

The Concatenation Operator

The concatenation "operator" is a period (full stop). It simply says "join this to this":

<?php
$parta = "This is part one";
$partb = " of a sentence!";
print $parta . $partb;
echo "<br>";
$msg = "PHP";
$msg .= " and MySQLi";   // .= appends to the variable
echo $msg;
?>

This is part one of a sentence!
PHP and MySQLi

Combined (Assignment) Operators

= is the assignment operator: $a = 5 assigns the value 5 to $a. It can be combined with other operators. If $a is 5, then after $a += 5 it is 10 – exactly as if you had written $a = $a + 5. Some programmers like combined operators because they reduce typing; use whichever form you find clearer.

<?php
$d1 = 12;
$d1 += 3; echo "After += 3 : $d1<br>";
$d1 -= 5; echo "After -= 5 : $d1<br>";
$d1 *= 2; echo "After *= 2 : $d1<br>";
$d1 /= 4; echo "After /= 4 : $d1<br>";
$d1 %= 3; echo "After %= 3 : $d1";
?>

After += 3 : 15
After -= 5 : 10
After *= 2 : 20
After /= 4 : 5
After %= 3 : 2

Other Useful Operators

• Ternary ?: – $result = (condition) ? value_if_true : value_if_false;
• Null coalescing ?? (PHP 7) – $name = $_GET['name'] ?? 'Guest'; gives the left value if it exists and is not null, otherwise the right value. Very handy for reading form fields.
• Error control @ – suppresses error messages of an expression (use rarely).

<?php
$marks = 72;
echo "Result: " . ($marks >= 40 ? "Pass" : "Fail") . "<br>";
$name = $_GET['name'] ?? 'Guest';
$city = $_GET['city'] ?? 'Unknown';
echo "Name: $name, City: $city";
?>

Result: Pass
Name: Ravi, City: Unknown

Precedence and Associativity

Think about this operation: $a = 7 - 4 + 1. If the minus happens before the plus, the result is 4; if the plus happens first, the result is 2. Since predictability is essential, an order of precedence was established, together with associativity.

• Precedence gives each operator a rank: operators with higher rank are evaluated first.
• When several operators have the same rank, associativity decides: left-associative operators are processed from left to right, right-associative operators from right to left. Some operators, such as the comparison operators, are non-associative.

The order of precedence of the operators in this chapter, from highest to lowest (operators on one line have equal precedence):

You can override precedence with parentheses – the innermost parentheses are processed first:

<?php
echo "7 - 4 + 1 = " . (7 - 4 + 1) . "<br>";
echo "(7 - 4) + 1 = " . ((7 - 4) + 1) . "<br>";
echo "7 - (4 + 1) = " . (7 - (4 + 1)) . "<br>";
echo "2 + 3 * 3 = " . (2 + 3 * 3) . "<br>";
echo "2 + (3 * 3) = " . (2 + (3 * 3)) . "<br>";
echo "(2 + 3) * 3 = " . ((2 + 3) * 3) . "<br>";
echo "1 + 3 + 3 * 2 = " . (1 + 3 + 3 * 2) . "<br>";
echo "1 + ((3 + 3) * 2) = " . (1 + ((3 + 3) * 2)) . "<br>";
echo "(1 + (3 + 3)) * 2 = " . ((1 + (3 + 3)) * 2);
?>

7 - 4 + 1 = 4
(7 - 4) + 1 = 4
7 - (4 + 1) = 2
2 + 3 * 3 = 11
2 + (3 * 3) = 11
(2 + 3) * 3 = 15
1 + 3 + 3 * 2 = 10
1 + ((3 + 3) * 2) = 13
(1 + (3 + 3)) * 2 = 14