How can regular string concatenation inside a large loop lead to performance problems, and what alternative approaches help avoid this issue when building up very large strings?

Repeatedly concatenating onto a string inside a loop that runs many thousands of times can become inefficient in some scenarios, since each concatenation may involve creating a new string in memory rather than simply extending the existing one, and for building up very large amounts of text, an alternative approach is to collect each individual piece into an array first, then join every element together at once using the implode function after the loop completes, which is typically both faster and clearer than building the same result through many repeated individual concatenation operations.

$parts = [];
foreach ($items as $item) {
    $parts[] = formatItem($item);
}
$result = implode("\n", $parts);