http://nl3.php.net/array_diff_assoc

"Two values from key => value pairs are considered equal only if (string) $elem1 === (string) $elem2 . In other words a strict check 
takes place so the string representations must be the same."

and 

"Note: This function only checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using, for 
example, array_diff_assoc($array1[0], $array2[0]);."

So values are casted to string for comparision.

The array-elements are casted to string, which results in "array". The following example returns no difference between the two arrays, 
while this is clearly not the case: http://3v4l.org/1LX4W#v540

Without notices, both arrays would look the same.  Notices are generated since 5.4.0, which is a good solution IMO.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

See the answer by arjen at react dot com.

The array_diff* methods aren't recursive. So you probably won't want to use them for multidimensional arrays, and thus it's an improvement that >= 5.4.0 warns you.

Thank you for your answers!
For a test I run this script on different versions of PHP 

<?php
echo "PHP version: ".phpversion()."<br />\n";
$source_packages = array(
	'one' => array ('param1' => 'First Parameter for One', 'param2' => 
'Second Parameter for One'),
	'two' => array ('param1' => 'First Parameter for Two', 'param2' => 
'Second Parameter for Two')
);
$packages_from = $source_packages;
var_dump($packages_from); echo "<br />\n";
$package_key = 'two';
$package_value = $source_packages[$package_key];
$packages_to = array($package_key => $package_value);
var_dump($packages_to); echo "<br />\n";
$result_packages = array_diff_assoc($packages_from, $packages_to);
var_dump($result_packages); echo "<br />\n";
?>
===================
PHP version: 5.3.10
array(2) { ["one"]=> array(2) { ["param1"]=> string(23) "First Parameter for 
One" ["param2"]=> string(24) "Second Parameter for One" } ["two"]=> array(2) { 
["param1"]=> string(23) "First Parameter for Two" ["param2"]=> string(24) 
"Second Parameter for Two" } }
array(1) { ["two"]=> array(2) { ["param1"]=> string(23) "First Parameter for 
Two" ["param2"]=> string(24) "Second Parameter for Two" } }
array(1) { ["one"]=> array(2) { ["param1"]=> string(23) "First Parameter for 
One" ["param2"]=> string(24) "Second Parameter for One" } } 
===================
PHP version: 5.4.3
array(2) { ["one"]=> array(2) { ["param1"]=> string(23) "First Parameter for 
One" ["param2"]=> string(24) "Second Parameter for One" } ["two"]=> array(2) { 
["param1"]=> string(23) "First Parameter for Two" ["param2"]=> string(24) 
"Second Parameter for Two" } }
array(1) { ["two"]=> array(2) { ["param1"]=> string(23) "First Parameter for 
Two" ["param2"]=> string(24) "Second Parameter for Two" } }
Notice: Array to string conversion in /usr/web/data/test_array_diff.php on line 
13 Notice: Array to string conversion in /usr/web/data/test_array_diff.php on 
line 13 array(1) { ["one"]=> array(2) { ["param1"]=> string(23) "First Parameter 
for One" ["param2"]=> string(24) "Second Parameter for One" } }
===================

Since 5.4+ version the work  of array_diff*  method changed. How I should 
rewrite this code to avoid errors? At the moment error log of a average visited 
site is increasing on 50+Mb every day because of this notice.

See http://nl3.php.net/manual/en/function.array-diff-assoc.php#73972

Thank you for answer!
It's solved.