Space
Space is the character: it is an invisible character. It serves as a separator between tokens, and is mostly unused by PHP execution engine.
Space is part of a class of invisible characters, called whitespaces, along with new line \n, carriage return \r, line feed \f and horizontal tabulation \t.
Spaces inside PHP code usually carry no value but to separate visually elements in the code. They are sometimes important, as in designing floats, or not important, with some operators, $a []= 1 is actually the same as $a[] = 1 or $a [ ] = 1.
Spaces outside the PHP tags are important, as they are delivered as is to the client. Any white space set before the opening <?php PHP tag may be send before PHP code is executed, leading to the infamous Cannot modify header information - headers already sent.
<?php
// space is mostly ignored in PHP code
$string = ' a ';
// space is important inside literal, such as strings
echo 1 . 2; // here, space is important
echo 1.2; // here, space is important
?>