No Flush In Loop

ID

php.no_flush_in_loop

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Php

Tags

efficiency, loop

Description

Reports a call to an output-flushing function (flush, ob_flush) inside a loop body. Flushing on every iteration sends the response to the client in many tiny chunks instead of one efficient block.

Rationale

flush() and ob_flush() push whatever has been buffered so far out to the web server and on to the client. Calling them once per iteration forces the server, and often the network and the browser, to handle a large number of small writes — each with its own overhead. Building the output during the loop and flushing a single time after it finishes does the same work with a fraction of the system calls.

<?php
foreach ($rows as $row) {
    echo render($row);
    flush();            // FLAW — flushes the buffer on every iteration
}

foreach ($rows as $row) {
    echo render($row);
}
flush();                // OK — flushed once, after the loop

Remediation

Move the flush() / ob_flush() call out of the loop so the buffered output is flushed once after the loop completes. Flush inside the loop only when you genuinely need incremental streaming (for example a long-running progress feed), and then consider flushing in batches.

References