Add PHP code after title in single post pages?

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You need to use the_title filter, not the_content.
Also make sure you have the_title() function somewhere in your single post page.
Here is the code:

function theme_slug_filter_the_content( $title ) {
    $custom_title="MY CODES";
    $custom_title  .= $title ;
    return $custom_title ;
}
add_filter( 'the_title', 'theme_slug_filter_the_content' );