Variables

Variables store values so you can use them later. In PHP every variable name starts with a dollar sign, you create a variable simply by assigning to it, and a variable can hold any type of value — PHP works out the type from the value. This lesson covers naming, assignment, checking whether variables exist, and how assignment copies values.

Creating variables

There's no separate declaration step: assigning a value creates the variable.

PHP 
<?php

$name = "Priya";
$age = 31;
$height = 1.68;
$isMember = true;

echo "$name is $age years old and $height m tall.\n";
Output
Priya is 31 years old and 1.68 m tall.

Variable names start with $, then a letter or underscore, followed by letters, digits or underscores. $total2 and $_count are fine; $2total isn't. Names are case-sensitive.

Changing values — and types

A variable can be reassigned at any time, even to a value of a different type. PHP is dynamically typed: the type belongs to the value, not the variable.

PHP 
<?php

$score = 10;
$score = $score + 5;
$score += 20;          // shorthand for $score = $score + 20
echo $score, "\n";

$value = 42;
var_dump($value);
$value = "forty-two";
var_dump($value);
Output
35
int(42)
string(9) "forty-two"

Changing a variable's type is legal, but it makes code harder to follow. Keep each variable to one kind of value; type declarations (lesson 29) help enforce that for function parameters and properties.

Undefined variables

Reading a variable that was never assigned gives a warning, and the value is treated as null:

PHP 
<?php

$total = $subtotal + 5;
echo $total, "\n";
Output
Warning: Undefined variable $subtotal in main.php on line 3
5

Most of the time this warning means a typo or a missing assignment. Fix the cause rather than hiding the warning.

isset, empty and unset

isset() checks whether a variable exists and isn't null; it never warns. empty() checks whether it's missing or "falsy" (such as 0, "" or null). unset() deletes a variable.

PHP 
<?php

$city = "Oslo";
$count = 0;
$nothing = null;

var_dump(isset($city), isset($nothing), isset($missing));
var_dump(empty($count), empty($city));

unset($city);
var_dump(isset($city));
Output
bool(true)
bool(false)
bool(false)
bool(true)
bool(false)
bool(false)

Be careful with empty(): the string "0" also counts as empty, which surprises people validating form input. Explicit checks like $x === '' are often clearer.

Null and the ?? operator

null means "no value". The null-coalescing operator ?? gives a fallback for a variable that is null or doesn't exist — without a warning:

PHP 
<?php

$nickname = null;
echo $nickname ?? "(no nickname)", "\n";
echo $undefinedVar ?? "(not set)", "\n";

$theme = null;
$theme ??= "light";    // assign only if null or unset
echo $theme, "\n";
Output
(no nickname)
(not set)
light

Assignment copies values

Assigning one variable to another copies the value. Changing one afterwards doesn't affect the other — even for arrays:

PHP 
<?php

$a = 10;
$b = $a;
$b = 99;
echo "a = $a, b = $b\n";

$original = [1, 2, 3];
$copy = $original;
$copy[] = 4;
echo count($original), " vs ", count($copy), "\n";
Output
a = 10, b = 99
3 vs 4

Objects are different: assigning an object copies a handle to the same object (lesson 33).

References

Putting & before the value makes a reference: two names for the same variable. Changing one changes the other:

PHP 
<?php

$price = 100;
$alias = &$price;
$alias = 80;
echo $price, "\n";
Output
80

References are occasionally useful (lesson 28 shows passing by reference), but they make code harder to reason about. Prefer returning new values.

Swapping and multiple assignment

Array destructuring (lesson 26) assigns several variables at once — including swapping two values without a temporary variable:

PHP 
<?php

[$x, $y] = [1, 2];
echo "$x $y\n";

[$x, $y] = [$y, $x];
echo "$x $y\n";
Output
1 2
2 1

Variable variables

PHP lets a variable's name come from another variable with $$. You'll see this in old code, but an array is almost always a better choice:

PHP 
<?php

$field = "colour";
$$field = "green";       // creates $colour
echo $colour, "\n";
Output
green

Try it yourself

Calculate the total cost of an order. Then try removing the $quantity line to see the warning.

Try it yourself · main.php
Edit the code, copy it into main.php, then run it with php main.php (PHP 8.5). How to install PHP
Output
4 x Notebook = 9.96
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