php - MySQL, CONCAT, the result is null sometimes

Im putting big data to table, to aLONGBLOB field. First I create the record:

INSERT INTO mytable VALUES (1, '');
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then in aforeach cycle I put the sliced data:

foreach (str_split($content, 1024*512) as $item)
{
    UPDATE mytable SET BIGCONTENT = CONCAT(BIGCONTENT, $item) WHERE ID = 1;
}
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yes, the field is growing, but after a few turn, (its about 1MB) it becomesnull! I tried another way:

UPDATE mytable SET BIGCONTENT = CONCAT_WS("", BIGCONTENT, $item) WHERE ID = 1;
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still the same. Whats the problem?