Created String concatenation (markdown)

String-concatenation.md

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+The concatenation operator should have one space on both sides in order to improve readability.
+
+<details><summary>Example</summary><div><br>
+
+**Bad**
+
+```PHP
+$text = $greeting.' '.$name.'!';
+```
+
+**Good** (add spaces)
+
+```PHP
+$text = $greeting . ' ' . $name . '!';
+```
+
+</div></details><br>
+
+You may break long lines into multiple lines using the concatenation operator. That way readability can improve considerable when combining lots of variables.
+
+<details><summary>Example</summary><div><br>
+
+**Bad**
+
+```PHP
+$text = $greeting.' '.$name.'!';
+```
+
+**Good** (split into multiple lines)
+
+```PHP
+$text = $greeting
+. ' '
+. $name
+. '!';
+```
+
+</div></details><br>
+
+_Reference_: [`Squiz.Strings.ConcatenationSpacing`](https://github.com/squizlabs/PHP_CodeSniffer/blob/master/src/Standards/Squiz/Sniffs/Strings/ConcatenationSpacingSniff.php)