diff --git a/build/code_sniffer/ruleset-php-legacy.xml b/build/code_sniffer/ruleset-php-legacy.xml
index 3a1d849135..621942b38b 100644
--- a/build/code_sniffer/ruleset-php-legacy.xml
+++ b/build/code_sniffer/ruleset-php-legacy.xml
@@ -63,6 +63,9 @@
  </rule>
  <rule ref="Squiz.Functions.FunctionDeclarationArgumentSpacing.SpacingAfterHint" />
 
+ <!-- All PHP built-in functions should be lowercased when called. -->
+ <rule ref="Squiz.Functions.LowercaseFunctionKeywords" />
+
  <!-- The eval() function MUST NOT be used. -->
  <rule ref="Squiz.PHP.Eval" />