Created Constants (markdown)

Constants.md

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+As in most languages, constants should be written in UPPERCASE.
+
+_Notice_: This does not apply to keywords!
+
+<details><summary>Example</summary><div><br>
+
+**Bad**
+
+```PHP
+const pi = 3.14;
+```
+
+**Good**
+
+```PHP
+const PI = 3.14;
+```
+
+</div></details><br>
+
+_Reference_: [`Generic.NamingConventions.UpperCaseConstantName`](https://github.com/squizlabs/PHP_CodeSniffer/blob/master/src/Standards/Generic/Sniffs/NamingConventions/UpperCaseConstantNameSniff.php)