WordPress Plugin Activation Error: Why Unexpected Output Appears and How to Fix It?

Unexpected Output During Plugin Activation: Causes and Solutions

When activating a WordPress plugin, it's common to encounter the message "The plugin generated X characters of unexpected output during activation." This error can be frustrating, especially when you're confident in your code's integrity.

Causes of Unexpected Output

There are two primary reasons why you might receive this error:

1. Output at the Wrong Location:

   • Output (e.g., echo) is mistakenly placed in the plugin activation function (register_activation_hook). Output should only occur within designated WordPress hooks, like admin_notices or the_content.

2. PHP Errors:

   • If there are underlying PHP errors in your plugin code, they can manifest during activation and trigger the error message.

Effective Solutions

1. Guard Against Incorrect Output:

• Wrap your activation function code in an if statement to suppress output outside of WordPress hooks:

function myPlugin( $post ) {
    if ( is_admin() && $pagenow !== 'plugins.php' ) {
        echo "No more alerts when its wrapped this way";
    }
}
register_activation_hook( __FILE__, 'myPlugin' );

2. Identify PHP Errors:

• Use the following code snippet to log errors during activation:

define('temp_file', ABSPATH.'/_temp_out.txt' );

add_action("activated_plugin", "activation_handler1");
function activation_handler1(){
    $cont = ob_get_contents();
    if(!empty($cont)) file_put_contents(temp_file, $cont );
}

add_action( "pre_current_active_plugins", "pre_output1" );
function pre_output1($action){
    if(is_admin() && file_exists(temp_file))
    {
        $cont= file_get_contents(temp_file);
        if(!empty($cont))
        {
            echo '<div class=&quot;error&quot;> Error Message:' . $cont . '</div>';
            @unlink(temp_file);
        }
    }
}

• If errors are logged in the _temp_out.txt file, debug your plugin code accordingly.

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