Make the check for arithmetic operators strict

Author: raxbg

src/Value/Value.php

@@ -162,7 +162,7 @@ public static function parsePrimitiveValue(ParserState $oParserState)
             try {
                 $oValue = self::parseIdentifierOrFunction($oParserState);
             } catch (UnexpectedTokenException $e) {
-                if (in_array($sNextChar, ['+', '-', '*', '/'])) {
+                if (in_array($sNextChar, ['+', '-', '*', '/'], true)) {
                     $oValue = $oParserState->consume(1);
                 } else {
                     throw $e;