Allow substr_compare() to take a negative offset to facilitate start of the

comparison from the end of string.

ext/standard/string.c

@@ -5770,6 +5770,10 @@ PHP_FUNCTION(substr_compare)
 		RETURN_FALSE;
 	}
 
+	if (offset < 0) { /* negative offset, start comparison at the end of string */
+		offset = s1_len + offset;
+	}
+
 	if (len && offset >= s1_len) {
 		php_error_docref(NULL TSRMLS_CC, E_WARNING, "The start position cannot exceed initial string length.");
 		RETURN_FALSE;