The isset() function is an built-in function in PHP which is used to check whether a variable is set or not. The isset() will return true if testing a variable is set and not null.

Example:

<?php
$var1 = '';
if(isset($var1)){
    echo '$var1 is set.';  // Output: $var1 is set.
}
echo "<br>";
 
$var2 = 'PHP Tutorials';
if(isset($var2)){
    echo '$var2 is set.';  // Output: $var2 is set.
}
echo "<br>";
 
// Unset the variable
unset($var2);
 
if(isset($var2)){
    echo '$var2 is set.';
} else{
    echo '$var2 is not set.';  // Output: $var2 is not set.
}
echo "<br>";
 
$var3 = NULL;
if(isset($var3)){
    echo '$var3 is set.';
} else{
    echo '$var3 is not set.';  // Output: $var3 is not set.
}
?>