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chapters/boolean-logic.md

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+Boolean logic is used to combine booleans to return another boolean.
+
+Using double ampersands tells PHP to check if both values are true.
+If so, it will return true. If not, it will return false.
+```php
+<?php
+
+$a = true;
+$b = true;
+$c = false;
+```
+
+Returns true.
+```php
+$a && $b;
+```
+Returns false.
+```php
+$a && $c;
+```
+
+Using two pipe characters checks if either value is true.
+Then, it will return true. If both values are false, then PHP
+returns false.
+```php
+$a = true;
+$b = false;
+$c = false;
+$d = true;
+```
+
+Returns true.
+```php
+$a || $b;
+```
+Returns false.
+```php
+$b || $c;
+```
+Returns true.
+```php
+$a || $d;
+```
+
+Using an exclamation point returns the value flipped.
+```php
+$d = true;
+```
+
+Outputs false.
+```php
+echo !$d;
+```