Hello, World

A PHP script starts with <?php; print with echo.

Runs in Zerethon IDE · PHP 8.4 Official PHP documentation ↗

main.php

<?php

echo "Hello, World!\n";
echo 'Sum: ' . (2 + 3) . PHP_EOL;
printf("%s has %d letters\n", 'PHP', 3);

Output

Hello, World!
Sum: 5
PHP has 3 letters

Here PHP runs on the command line (CLI), not in a web server: output goes to the Output tab, there is no browser, $_GET or HTML page.
echo does not add a newline — end with "\n" or PHP_EOL.
Join strings with ., not +. Double quotes expand variables and \n; single quotes do not.
A file with only PHP code needs no closing ?>.

Official references