Robert Cummings wrote:
Then how come when I do a foreach on an array (without modifying
anything within the foreach), it still makes a copy of the array that
consumes memory? I think it's dangerous to generalize that it's always
best to let PHP make copies of things. In the foreach situation, the
preferred solution when memory is a problem is to either use a
reference, or have foreach iterate over the keys of the array.
Regards, Adam.
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PHP doesn't seem to make a real copy of data until one of the copies is
modified, making it necessary to create a new set of data. So, it is pretty
smart about that.
Here is a small CLI script that seems to support this:
<?php
$a_array = array();
for($i = 0; $i < 10000; $i++) {
$a_array[] = time(); // Just an arbitrary piece of data
}
echo 'Memory Usage: ' . memory_get_usage() . "\n";
echo "Making a copy of the array.\n";
$a_copy = $a_array;
echo 'Memory Usage: ' . memory_get_usage() . "\n";
echo "Modifying the copy:\n";
$a_copy[] = time();
echo 'Memory Usage: ' . memory_get_usage() . "\n";
?>
On my machine, this displays:
Memory Usage: 640280
Making a copy of the array.
Memory Usage: 640440
Modifying the copy:
Memory Usage: 1106056
-K. Bear
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