Test for the minimum file permissions required for the webserver to read a file that it has written

perm-test.php

<?php
function perm_test() {
    $file = '.perm_test';
    @touch( $file );
    if ( ! is_readable( $file ) )
        return false;

    $perms = array( 0666, 0664, 0644, 0660, 0640, 0600, 0444, 0440, 0400 );

    $lastperm = 0666;
    foreach ( $perms as $perm ) {
        if ( ! @chmod( $file, $perm ) )
            return false;

        if ( ! is_readable( $file ) )
            return $lastperm;

        $lastperm = $perm;
    }

    @unlink( $file );
    return $lastperm;
}

var_dump(decoct(wp_perm_test()));