Why can't users login in version 1.5.7 with php 4?

As reported by auciker, because of a change made to improve security, login with redirection will not work for PHP 4 sites. This may also impact other areas where there is redirection, specifically polls and mailto.

To fix this:

• Add this to the end of /libraries/joomla/utilities/compat/php50x.php

if (!function_exists( 'stripos' )) {
    function stripos( $haystack, $needle, $offset = 0 ) {
        return strpos( strtolower( $haystack ), strtolower( $needle ), $offset );
    }
}

• OR - Upgrade to PHP 5. PHP 4 end of life was August 8, 2008. There will not be any more PHP 4 releases, even for security. (Switching PHP versions may cause problems with some 3rd Party Plugins - be sure to test thoroughly).