If a required field is left empty in a PHP form, you can display an error message by ______.

If a required field is left empty in a PHP form, you can display an error message by assigning an error message to a variable and then displaying it to the user. When the form is submitted, you can check if the required field is empty. If it is empty, you can assign an appropriate error message to a variable, and then display the error message in a visible location on the form, such as next to the field or at the top of the form. This provides feedback to the user, informing them about the missing required field and prompting them to fill it in. Learn more: https://www.php.net/manual/en/tutorial.forms.php