Description:
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The multibyte split function (function.mb_split) does not appear to be working as expected. 

Instead of parsing and converting the multibyte string into array elements as specified, the character elements are being split at unpredictable positions, yielding unpredictable array elements.

The "Test script" demonstrates an expected result for the first multibyte string, but then shows unexpected results for the next two multibyte strings.

Thank you for reviewing this potential bug.

Test script:
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<?php
header('Content-Type: text/html; charset=UTF-8');
mb_regex_encoding('UTF-8');
mb_internal_encoding('UTF-8');

$arr = mb_split('\B', "你好"); # Array ( [0] => 你 [1] => 好 ) ## Okay!
print_r($arr);

$arr = mb_split('\B', "你你"); # Array ( [0] => 你 [1] => 你 ) ## Expected Result
print_r($arr); ## Instead, this message appears:
## Warning: mb_split(): mbregex search failure in mbsplit():
## no support in this configuration in /dir/foo.php on line 22

$arr = mb_split('\B', '隨著劇情的推進');
print_r($arr);
# Expected Result
# Array ( [0] => 隨 [1] => 著 [2] => 劇 [3] => 情 [4] => 的 [5] => 推 [6] => 進 )

# Actual Result, NOT expected
# Array ( [0] => 隨 [1] => � [2] => �劇 [3] => � [4] => �的 [5] => � [6] => �進 )