add_filter doesn't work

Question

I'm developing a plugin to replace a character for some user agents in titles, posts and comments but the filter doesn't work. Here is the plugin:

<?php
/*
Plugin Name: WP-HalfSpace
Description: نمایش فاصله به‌جای نیم‌فاصله برای گوگل برای خراب‌نشدن نوشته در نتایج جست‌وجوی گوگل
Version: 1.0.0
Author: ahmadalli
Author URI: http://2barnamenevis.com
*/
function HalfSpace_filter($content)
{

    $u_agent = $_SERVER['HTTP_USER_AGENT'];
    echo "hi";
    if(($u_agent=="Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" )
    || ($u_agent=="Googlebot/2.1 (+http://www.googlebot.com/bot.html)")
    || ($u_agent=="Googlebot/2.1 (+http://www.google.com/bot.html)")
    || ($u_agent=="Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)")
    )
    { 
        echo "aaaaaaaaa";
        $content=str_ireplace("‌‌"," ",$content); //"" is not empty. it's a persian character names halfspace.
    }
    return $content;
}

register_activation_hook( __FILE__, 'HalfSpacePlugin_activate' );

function HalfSpacePlugin_activate()
{
    add_filter('the_title','HalfSpace_filter',1,1);
    add_filter('the_content','HalfSpace_filter',1,1);
    add_filter('comment_text','HalfSpace_filter',1,1);
}



register_deactivation_hook( __FILE__, 'HalfSpacePlugin_deactivate' );

function HalfSpacePlugin_deactivate()
{
    remove_filter('the_title','HalfSpace_filter');
    remove_filter('the_content','HalfSpace_filter');
    remove_filter('comment_text','HalfSpace_filter');
}
?>

The filter doesn't work at all because echo "hi"; in HalfSpace_filter doesn't do it's job!

Can anybody help me?

Answer 1

register_activation_hook and register_deactivation_hook refer to when the plugin is activated and deactivated, respectively, and only fire in those two instances. They're to be used, for example, when you need to add a database table (and take it back down). You don't need to remove your filters, either. Your plugin can just be shortened to:

function HalfSpace_filter( $content ) {
    $u_agent = $_SERVER['HTTP_USER_AGENT'];
    echo "hi";
    if ( ( $u_agent=="Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" )
    || ( $u_agent=="Googlebot/2.1 (+http://www.googlebot.com/bot.html)" )
    || ( $u_agent=="Googlebot/2.1 (+http://www.google.com/bot.html)" )
    || ( $u_agent=="Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)" )
    ) { 
        echo "aaaaaaaaa";
        $content = str_ireplace( "‌‌", " ",$content ); //"" is not empty. it's a persian character names halfspace.
    }
    return $content;
}
add_filter( 'the_title','HalfSpace_filter' );
add_filter( 'the_content','HalfSpace_filter' );
add_filter( 'comment_text','HalfSpace_filter' );

Answer 2

Activation and deactivation handlers are called during plugin activation and deactivation only. Hence the name. Just remove that condition, and it will work.