PHP
Checking variables with isset() and unset()
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.
Next lesson: Summary - Variables
