Numbered basics chapters for sortability

chapters/basics/04-strings.md

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+As seen in the first chapter, a string is a group of characters created by
+surrounding text in single or double quotes.
+```php
+<?php
+
+$firstname = 'Joey';
+$lastname = "Johnson";
+```
+
+A double quoted string can interpret special characters starting
+with a back slash to create formatting. The `\n` creates a newline
+between the names and after them.
+```php
+echo "Jacob\nJones\n";
+```
+
+Double quoted strings can also embed variables in the text. This code
+outputs "Cindy Smith".
+```php
+$firstname = 'Cindy';
+echo "$firstname Smith\n";
+```
+
+Another feature of strings is the ability to combine them together.
+To combine two strings, use the period character in between them.
+```php
+$firstname = 'Jenny';
+$lastname = 'Madison';
+$fullname = $firstname . $lastname;
+echo $fullname;
+```