Dear All,
I had always thought of concatenating assignment and concatenation + assignment to the
same variable
as really just syntatical variations i.e. I thought that
$data = $data . <some strings>;
and
$data .= <some strings>;
were just alternate syntaxes for the same operation. I've always tended to use the
long format on
the grounds that it was more readable and maintainable.
How wrong I was! It seems the performance on big strings can be hugely different. I
think I know
why but I'd appreciate confirmation.
I came across this when investigating a performance issue with writing out a
gz-encoded csv file
from an SQL table. The code is something like:
$data = '';
while ($row_product = mysql_fetch_array($cur_product))
{
$data = $data .
'"'.str_pad(strip_tags(strtr($row_product[product_code],'"\',',' ')),40)
.'","'.str_pad(strip_tags(strtr($row_product[product_name],'"\',',' ')),60)
.'","'.str_pad(strip_tags(strtr($row_product[product_desc],'"\',',' ')),120)
.'"'."\r\n";
}
$Size = strlen($data);
$Crc = crc32($data);
$data = gzcompress($data);
$data = "\x1f\x8b\x08\x00\x00\x00\x00\x00" . substr($data,0,strlen($data) -4)
. pack("V",$Crc). pack("V",$Size);
fwrite($handle,$data);
fclose($handle);
There seemed to be plenty of reasons why this ran slow (5 seconds plus on only a
couple of thousand
product rows). I suspected each of the strtr, strip_tags, str_pad and gzcompress in
turn but it
turned out that a simple change:-
$data = $data .
into
$data .=
ran an order of magnitude faster (i.e. less than 0.5s).
I guess that in the first case a working copy of $data has to be made, whereas in the
second, the
concatenation is done directly on the existing copy of data i.e. the performance
difference is just
the price of creating and throwing away two thousand copies of $data.
Does that make sense? Anyone know of other cases where alternate syntaxes can make
such a
difference to performance?
If I get some confirmation of this analysis I'll bung a note on the manual at
http://www.php.net/manual/en/language.operators.string.php
Humbled,
George
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