LowExpiryCacheTime: improve handling of potential parse errors in evaled code

Issue reported by GaryJones in Automattic/VIP-Coding-Standards 628#issuecomment-790653774

In rare cases, it is possible that the call to `eval()` with "safe" tokens only, would still result in a parse error.

An example of such a case is when the PHP 5.6 `**` (`T_POW`) operator is used. PHPCS backfills the tokenization, which means that the operator is correctly recognized by the sniff as a "safe" token to use in the `eval()` statement.

However, when the sniff is being run on PHP 5.4/5.5, the `eval()` will also be run on PHP 5.4/5.5 and while PHPCS backfills the token, PHP does not, resulting in a parse error in the `eval`-ed code, upon which the `eval()` will return `false`.

This commit:
* Silences the parse error notice.
* Checks the output of the `eval()` for it being boolean `false` and if so, flags the statement for manual inspection, same as when any "non-safe" token is encountered.

Author: jrfnl

WordPressVIPMinimum/Sniffs/Performance/LowExpiryCacheTimeSniff.php

@@ -77,6 +77,10 @@ public function process_parameters( $stackPtr, $group_name, $matched_content, $p
 		$reportPtr      = null;
 		$openParens     = 0;
 
+		$message    = 'Cache expiry time could not be determined. Please inspect that the fourth parameter passed to %s() evaluates to 300 seconds or more. Found: "%s"';
+		$error_code = 'CacheTimeUndetermined';
+		$data       = [ $matched_content, $parameters[4]['raw'] ];
+
 		for ( $i = $param['start']; $i <= $param['end']; $i++ ) {
 			if ( isset( Tokens::$emptyTokens[ $this->tokens[ $i ]['code'] ] ) === true ) {
 				$tokensAsString .= ' ';
@@ -151,9 +155,7 @@ public function process_parameters( $stackPtr, $group_name, $matched_content, $p
 			}
 
 			// Encountered an unexpected token. Manual inspection needed.
-			$message = 'Cache expiry time could not be determined. Please inspect that the fourth parameter passed to %s() evaluates to 300 seconds or more. Found: "%s"';
-			$data    = [ $matched_content, $parameters[4]['raw'] ];
-			$this->phpcsFile->addWarning( $message, $reportPtr, 'CacheTimeUndetermined', $data );
+			$this->phpcsFile->addWarning( $message, $reportPtr, $error_code, $data );
 
 			return;
 		}
@@ -177,7 +179,17 @@ public function process_parameters( $stackPtr, $group_name, $matched_content, $p
 			}
 		}
 
-		$time = eval( "return $tokensAsString;" ); // phpcs:ignore Squiz.PHP.Eval -- No harm here.
+		$time = @eval( "return $tokensAsString;" ); // phpcs:ignore Squiz.PHP.Eval,WordPress.PHP.NoSilencedErrors -- No harm here.
+
+		if ( $time === false ) {
+			/*
+			 * The eval resulted in a parse error. This will only happen for backfilled
+			 * arithmetic operator tokens, like T_POW, on PHP versions in which the token
+			 * did not exist. In that case, flag for manual inspection.
+			 */
+			$this->phpcsFile->addWarning( $message, $reportPtr, $error_code, $data );
+			return;
+		}
 
 		if ( $time < 300 && (int) $time !== 0 ) {
 			$message = 'Low cache expiry time of %s seconds detected. It is recommended to have 300 seconds or more.';

WordPressVIPMinimum/Tests/Performance/LowExpiryCacheTimeUnitTest.php

@@ -55,6 +55,7 @@ public function getWarningList() {
 			77  => 1,
 			78  => 1,
 			79  => 1,
+			82  => ( PHP_VERSION_ID > 50600 ) ? 0 : 1,
 			88  => 1,
 			94  => 1,
 			95  => 1,