Whitespace
Whitespace refers to spaces, tabulations, and newline characters. Those characters are invisible on a white background, hence the name.
Whitespace is used to format and organize the code, making it more readable.
However, whitespace is generally not significant in terms of parsing; the PHP interpreter usually ignores whitespace. There are some rare situations where the whitespaces have a role: in heredoc syntax, and for formatting floats.
Whitespace is not a notion specific to PHP: other languages have them too. For example, trim() removes whitespace at the beginning and the end of a string value.
<?php
$heredoc = <<<HEREDOC
One tab shift
Here
HEREDOC;
$concatenation = 1 . 0; // This is 10
$float = 1.0; // This is 1.0
//$invalid = 1. 0; // This is a syntax error
?>