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In PHP development, ob_flush() and flush() are commonly used functions to flush the output buffer. By default, PHP stores output content in an internal buffer until the script finishes executing or the buffer is manually flushed, at which point the content is sent to the browser.
ob_flush() is used to flush the output buffer and immediately send the buffered content to the browser. flush() achieves a similar effect, but in some server environments, additional steps may be necessary to ensure the flush takes effect.
ob_flush() primarily flushes PHP's output buffer, sending all buffered data, including HTTP headers and HTML content, ensuring the browser receives the data as soon as possible.
The flush() function sends the current buffer's data to the browser but does not output HTTP header information. Compared to ob_flush(), flush() is faster because it bypasses processing HTTP headers.
Typically, ob_flush() and flush() are used together to achieve immediate content output. However, some servers require calling ob_end_flush() to close the buffer before flush() can take effect.
The following example shows how to use ob_flush() and flush():
<?php
// Start output buffering
ob_start();
// Output part of the content
echo "Hello";
// Immediately flush buffer and send to browser
ob_flush();
flush();
// Output remaining content
echo "World";
// Close output buffer
ob_end_flush();
?>
In this code, output buffering is enabled with ob_start(). After outputting "Hello", ob_flush() and flush() are called to send content immediately to the browser. Then "World" is output, and finally the buffer is closed with ob_end_flush(). This allows chunked output to improve user experience.
Some server environments may have output buffering enabled or disabled by default. When using these functions, you should verify if the server supports immediate flush and adjust configuration or call ob_end_flush() if necessary.
Not all browsers support real-time flushing of buffers, so even if flushed on the server side, browsers might still delay displaying content.
When flushing buffers, be mindful of the output order. Content output first will be displayed first, and subsequent ob_flush() calls append new content after the previous output.
ob_flush() and flush() are important PHP functions for controlling output buffer flushing behavior. Proper use of them can lead to smoother page loading experiences, but must be combined with server configuration and browser characteristics.
