What are common mistakes when trying to run a simple "Hello World" PHP code and how can they be avoided?

One common mistake when running a simple "Hello World" PHP code is forgetting to include the opening `<?php` tag at the beginning of the code. Another mistake is not using the `echo` statement to output the text. To avoid these mistakes, always start your PHP code with `<?php` and use `echo` to display the text.

<?php
echo "Hello World!";
?>