How to insert a custom handler in the output cache chain

In PHP, the mechanism allows you to intercept the data output by the script, modify it or delay sending it. Usually, we will use ob_start() to enable cache and customize the processing of the output content by passing in the callback function.
The ob_list_handlers() function can be used to view all current activity output handlers.

This article will use practical examples to demonstrate how to understand the cache chain through the ob_list_handlers function and insert its own handler in it.

1. Understand ob_list_handlers

ob_list_handlers() returns an array listing all currently activated output buffer processors. For example, if you have enabled the gzip compression module, you may see a processor name like ob_gzhandler .

Example:

 <?php
// Turn on the default output cache
ob_start();

// Check the current output handler
print_r(ob_list_handlers());

ob_end_flush();
?>

The output may be similar to:

 Array
(
    [0] => default output handler
)

If there is no other custom processor, you usually only see the default output handler .

2. Insert custom output processor

We can use ob_start() and pass in a custom callback function, which will be processed before the output is sent to the browser.

Here is a simple example: We define a processor that turns all output lowercase letters to uppercase.

 <?php
// Define custom handlers
function custom_uppercase_handler($buffer) {
    // Process the output content，For example, turn all lowercase letters into uppercase
    return strtoupper($buffer);
}

// Insert custom handler in cache chain
ob_start('custom_uppercase_handler');

// Analog output
echo "Welcome to visit https://gitbox.net Test page！";

ob_end_flush();
?>

Output result:

 Welcome to visit HTTPS://GITBOX.NET Test page！

As you can see, all lowercase letters have been converted to uppercase, including the domain name gitbox.net in the URL.

3. Insert multiple processors

PHP's output cache supports multi-layer nesting. You can call ob_start() multiple times, and each layer can specify a different processing function.

Example:

 <?php
// The first handler：Convert content to capital
function handler_upper($buffer) {
    return strtoupper($buffer);
}

// The second handler：Replace specific words
function handler_replace($buffer) {
    return str_replace('GITBOX.NET', 'gitbox.net', $buffer);
}

// Turn on the first processor
ob_start('handler_upper');

// Turn on the second processor
ob_start('handler_replace');

// Output content
echo "Welcome to visit https://gitbox.net Test page！";

// Turn off the top processor and output
ob_end_flush();

// Turn off the lower processor
ob_end_flush();
?>

In this example:

1. The handler_replace runs first and replaces the string.

2. After running handler_upper , the processed content is converted to capitalization.

So the final output is:

 Welcome to visit HTTPS://gitbox.net Test page！

Note that handler_replace runs before handler_upper , so replacement operations are not affected by uppercase conversion.

4. Dynamic detection combined with ob_list_handlers()

You can use ob_list_handlers() to dynamically check the output processing chain in the program, such as determining whether the custom processor is inserted correctly during debugging.

 <?php
ob_start('custom_uppercase_handler');

$handlers = ob_list_handlers();
print_r($handlers);

ob_end_flush();
?>

Output:

 Array
(
    [0] => custom_uppercase_handler
)

Instructions that the custom processor has been inserted successfully.

Summarize

• Use ob_start() to insert custom handlers into the output cache chain.

• ob_list_handlers() is used to view all currently active cache processors and helps debug cache chains.

• Ob_start() can form a multi-layer processing chain, paying attention to the order of processor calls.

• When processing output, remember to be careful about the encoding format and case changes of the content to avoid causing unexpected problems.

By rationally using the output cache mechanism, the flexibility of PHP programs can be greatly enhanced, such as implementing content filtering, dynamic compression, delayed output and other functions.