Empty String
The empty string is a string without any content. It may be built with single quotes, double quotes or the Heredoc and NowDoc syntaxes.
The empty string is the neutral element for concatenation: concatenating an empty string to another string does not change the latter.
The empty string may be used as an error report: for example, substr() returns an empty string when the offset is beyond the string range.
The empty string often serves as a default value, for properties and variables.
Comparisons between '' and other falsy values, such as 0, changed with PHP 8.0: it was true and became false.
The empty string is falsy: it is converted to the false boolean when implicit conversion is needed, such as in a condition.
<?php
$emptyString1 = ''; // single quote
$emptyString2 = ""; // double quote
$emptyString3 = <<<STRING
STRING;
$emptyString4 = <<<'STRING'
STRING;
?>