Checking variables with isset() and unset()

PHP course · Beginner · Lesson 11/116

Learn to check a variable's existence with isset() and remove it with unset().

Checking variables with isset()

The isset() function asks whether a variable exists. It returns true if it is defined, false otherwise.

Example: you declared $name = "Alex";. echo isset($name); prints 1 (true appears as 1 with echo).

For an undefined variable, echo isset($missing); prints an empty string (false appears as empty with echo).

This function is handy when you want to check a variable exists before using it.

Removing a variable with unset()

The unset() function completely destroys a variable. After that, no variable with that name exists.

Example: you declared $city = "Berlin";. After unset($city);, $city is gone.

If you then write echo isset($city);, the output is empty; the variable was removed and isset() returns false.

Once you remove a variable with unset(), PHP no longer knows it until you define it again.

Exercise Check a variable with isset

Assign "Berlin" to $city. Write echo "Before: " . isset($city) . "\n";. Then remove it with unset($city). Write echo "After: " . isset($city) . "\n";. Expected output: Before: 1 and After: (empty).

Show example solution

Write the Before line, then unset, then the After line.
Example solution:
<?php
$city = "Berlin";
echo "Before: " . isset($city) . "\n";
unset($city);
echo "After: " . isset($city) . "\n";

Test yourself

What does $name = "Alex"; isset($name) return?
```
$name = "Alex";
echo isset($name);
```
- The variable's value: "Alex"
- true
- false
- null
Show answer

true — Correct! isset returns true for a defined variable.
Write the function to delete a variable
```
____($city);
```
Show answer
```
unset($city);
```
Match each function to its task

isset()

asks if it exists
unset()

deletes the variable
echo

prints the value
=

assigns a value
What does $city = "Hamburg"; unset($city); echo isset($city); print?
```
$city = "Hamburg";
unset($city);
echo isset($city);
```
- "Hamburg"
- 1
- true
- empty (prints nothing)
Show answer

empty (prints nothing) — Correct! unset removes the variable; isset returns false, echo prints empty.

Checking variables with isset() and unset()