PHP Output Control

PHP Output Control allows developers to control what gets sent as output to the browser. This can be especially useful for buffering output or setting headers after some content has been generated. Here are the primary aspects of PHP's Output Control:

Output Buffering: When you turn on output buffering, no output is sent from the script (other than headers), instead, the output is stored in an internal buffer.

• ob_start(): Turn on output buffering.
• ob_get_contents(): Return the contents of the output buffer.
• ob_end_clean(): Clean (erase) the output buffer and turn off output buffering.
• ob_end_flush(): Send the contents of the output buffer and turn off output buffering.

ob_start(); // Start buffering

echo "Hello, World!"; // This won't be immediately sent to the browser

$output = ob_get_contents(); // Get the buffer content

ob_end_clean(); // Clean the buffer

echo $output; // Outputs "Hello, World!"

Flushing the Output: In some cases, you might want to flush part of the buffered output to the browser.

• ob_flush(): Flush the output buffer, but keep buffering active.
• flush(): This tries to push current output all the way to the browser.

ob_start(); // Start buffering

echo "Part 1 of my content.";
ob_flush(); // Flush this part to the browser
flush(); // Try to push it all the way to the browser

sleep(2); // Wait for 2 seconds

echo "Part 2 of my content.";

ob_end_flush();

Nested Output Buffering: You can have nested output buffering, meaning you can start a new buffer while another one is still active. Each ob_start() call needs its own ob_end_clean() or ob_end_flush().

ob_start();
echo "First level.";

ob_start();
echo "Second level.";
$second_level = ob_get_contents();
ob_end_clean();

ob_end_flush();

echo $second_level; // Outputs "Second level."
   

Output Control Functions:

• ob_get_level(): Return the nesting level of the output buffering mechanism.
• ob_get_length(): Return the length of the output buffer.
• ob_get_clean(): Get current buffer contents and delete current output buffer.

ob_start();
echo "Hello, World!";

if (ob_get_level() > 0) {
    echo "\nBuffering is active.";
    echo "\nBuffer length: " . ob_get_length();
}

$content = ob_get_clean();
echo $content; // Outputs everything

Output Handlers: 

PHP allows for the possibility to use callback functions with the output buffer. This lets you process the buffer's content before it's sent to the browser.

function callback($buffer) {
    // Convert all the content to uppercase
    return strtoupper($buffer);
}

ob_start("callback");

echo "This will be uppercased.";

ob_end_flush(); // Outputs "THIS WILL BE UPPERCASED."