In what ways can PHP developers optimize their code to improve performance and avoid common errors like undefined variables?

One way PHP developers can optimize their code to improve performance and avoid common errors like undefined variables is by using isset() or empty() functions to check if a variable is set before using it in their code. This helps prevent errors that can occur when trying to access variables that have not been initialized or assigned a value.

// Check if a variable is set before using it
if(isset($variable)){
    // Use the variable here
    echo $variable;
}