What potential issue arises when using if statements with PHP variables that are not properly defined?

Using if statements with PHP variables that are not properly defined can lead to errors such as "Undefined variable" or "Undefined index". To avoid these issues, you can use the isset() function to check if a variable is set before using it in an if statement. This way, you can prevent errors and ensure that your code runs smoothly.

if(isset($variable)){
    // do something with $variable
} else {
    // handle the case where $variable is not set
}