The eval() function in PHP allows for dynamic execution of code passed as a string.

Echo and print are not exactly the same; echo can take multiple parameters, while print only takes one and returns a value (1).

The PUT method is idempotent, meaning that multiple identical requests will have the same effect as a single request. It is typically used for updating a resource in RESTful web services, as it replaces the current representation of the target resource with the new data provided in the request.

In HTML5, adding the 'required' attribute to input fields ensures that the form cannot be submitted if those fields are left empty. The browser will automatically prompt the user to fill in the required fields before allowing submission.

The meta charset="UTF-8" tag should be placed within the head tag of an HTML document. It specifies the character encoding for the HTML document, ensuring that the browser correctly renders the text content.

The 'defer' attribute in a script tag ensures that the script is executed only after the HTML document has been fully parsed. This is useful for scripts that rely on the complete DOM being available before they run.

Media queries in CSS are used to apply different styles to a website depending on the characteristics of the user's device, particularly the screen size. This technique is essential for creating responsive web designs that provide an optimal viewing experience across various devices.