How to free memory after array in PHP? unset and = null seems to not working

How to free memory after array in PHP? unset and = null seems to not working

I ve got a 16 MB size CSV file and try to parse it, and do some stuff, but
script run out of memory after some time. I realize that this code
generate around 200 MB of used space and unset is not working.
$countRows = 1;
var_dump("3. ".memory_get_usage()." beginDiff: ".(memory_get_usage() -
$this->startingMemory));
while(($row = fgetcsv($fp, 300000, ';', '"')) !== FALSE)
{
if ($row == '')
continue;
if($firstRow == true)
{
foreach($row as $k => $v)
{
$this->columnMapping[$k] = trim(mb_strtolower($v));
}
$firstRow = false;
continue;
}else
{
foreach($row as $k => $v)
{
$row[$this->columnMapping[$k]] = $v;
unset($row[$k]);
}
}
...
//$this->theirCategoriesToProducts[$row['kategorie']][]['kodproduktu']
= $row['kodproduktu'];
$this->theirCategoriesToProducts[$row['kategorie']][] = $row;
}
var_dump("3,5. ".memory_get_usage()." beginDiff:
".(memory_get_usage() - $this->startingMemory));
...
var_dump("7. - before unset total: ".memory_get_usage()." beginDiff:
".(memory_get_usage() - $this->startingMemory));
unset($this->theirCategoriesToProducts);
var_dump("8. - after unset total: ".memory_get_usage()." beginDiff:
".(memory_get_usage() - $this->startingMemory));die;
Generating this output:
string '3. 72417440 beginDiff: 34730040' (length=31)
string '3,5. 292748528 beginDiff: 255061136' (length=36)
string '7. - before unset total: 299039360 beginDiff: 261351984'
(length=55)
string '8. - after unset total: 297364432 beginDiff: 259677056'
(length=54)
With setting that variable equals null is the output very similar. But
switching comments between this two lines
$this->theirCategoriesToProducts[$row['kategorie']][]['kodproduktu'] =
$row['kodproduktu'];
//$this->theirCategoriesToProducts[$row['kategorie']][] = $row;
Will output:
string '3. 72417784 beginDiff: 34730040' (length=31)
string '3,5. 81081984 beginDiff: 43394248' (length=34)
string '7. - before unset total: 87256544 beginDiff: 49568824'
(length=53)
string '8. - after unset total: 85581520 beginDiff: 47893800' (length=52)
So its about 200 MB of "lost" memory (almost half of dedicated).
Recursive function unseting all parts of arrays eat more memory, than was
able to free so crashed also.
In script is never used that array with & so there should be no references
to other variables.
File is closing right after 3.5 dump.
Any other ideas, how to unset that array?