Common Mistake in Echo statement

I am describing a little problem, Normally face by a programmer.
This is a common Problem, Which can Produce in a large program a wrong output.

So,

You know the use of echo in PHP, which used for displaying data or some result in the webpage.

Example are as follow:
<?php

//   (1)  for displaying some paragraph;
echo "I am writing a code.";
// it will display the line on the webpage:  I am writing a code.

//   (2) For addition of Numbers
echo 3+3;
// It will display on the webpage: 6

//   (3)  Paragraph and addition of numbers
echo "The Sum is : 3 + 3 :". 3 + 3;

// it will display on the webpage:  6

?>