Echo

echo is a language construct used to output text to the browser or the output stream. It’s commonly used to display content on a web page, generate HTML, or provide feedback to users. The echo construct is often used to generate dynamic content that is sent to the client’s web browser.

echo is a language construct of PHP, not a function: it may be not be used as a callback.

echo works without parenthesis: then, it accepts an arbitrary number of arguments.

<?php

    echo 'Hello', ' ', 'world!';

?>

Documentation

See also The difference between echo and print in PHP.

Related : Print, Language Construct, php://output, print_r(), printf(), STDOUT