Reading []
Class and object changes
Migrating from PHP 5.0.x to PHP 5.1.x
PHP Manual

## Integer values in function parameters

With the advent of PHP 5.0.x, a new parameter parsing API was introduced which is used by a large number of PHP functions. In all versions of PHP between 5.0.x and 5.1.x, the handling of integer values was very strict and would reject non-well formed numeric values when a PHP function expected an integer. These checks have now been relaxed to support non-well formed numeric strings such as " 123" and "123 ", and will no longer fail as they did under PHP 5.0.x. However, to promote code safety and input validation, PHP functions will now emit an `E_NOTICE` when such strings are passed as integers.

Reading []
Class and object changes
Migrating from PHP 5.0.x to PHP 5.1.x
PHP Manual