Fixed ValueError message in substr_compare()

It used some random argument name instead of

Author: Girgias

ext/standard/string.c

@@ -5960,7 +5960,7 @@ PHP_FUNCTION(substr_compare)
 	}
 
 	if ((size_t)offset > ZSTR_LEN(s1)) {
-		zend_argument_value_error(3, "must be contained in argument #1 ($main_str)");
+		zend_argument_value_error(3, "must be contained in argument #1 ($haystack)");
 		RETURN_THROWS();
 	}
 

ext/standard/tests/strings/bug40754.phpt

@@ -81,7 +81,7 @@ string(6) "abcdex"
 int(0)
 int(0)
 substr_count(): Argument #3 ($offset) must be contained in argument #1 ($haystack)
-substr_compare(): Argument #3 ($offset) must be contained in argument #1 ($main_str)
+substr_compare(): Argument #3 ($offset) must be contained in argument #1 ($haystack)
 stripos(): Argument #3 ($offset) must be contained in argument #1 ($haystack)
 substr_count(): Argument #3 ($offset) must be contained in argument #1 ($haystack)
 substr_count(): Argument #4 ($length) must be contained in argument #1 ($haystack)