Created Keywords (markdown)

Keywords.md

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+`true`, `false` and `null` must be written in lower case letters.
+
+<details><summary>Example</summary><div><br>
+
+**Bad**
+
+```PHP
+if($condition === TRUE && $error === FALSE) {
+    return NULL;
+}
+```
+
+**Good**
+
+```PHP
+if($condition === true && $error === false) {
+    return null;
+}
+```
+
+</div></details><br>
+
+_Reference_: [`Generic.PHP.LowerCaseConstant`](https://github.com/squizlabs/PHP_CodeSniffer/blob/master/src/Standards/Generic/Sniffs/PHP/LowerCaseConstantSniff.php)