Passing global variable from header.php to functions.php

Question

Situation:

Is header.php file where is code: if(is_single()){ global $my_global_var; $my_global_var == true; }

Is functions.php file where is code: global $my_global_var; if($my_global_var == true) { echo "OK"; }

But this is not working, what im doing wrong?

In sidebar all is fine.

Fixed 0.2: Must be:

    add_filter('the_content','simple_img_tag_search');
    function simple_img_tag_search($content){
    if(is_single()){
        $MYCODE = "<div>My code</div>";
        $paragraphs = explode( '</h2>', $content, 2 );
        $first_paragraph = $paragraphs[0];
        return str_replace($first_paragraph.'</h2>',$first_paragraph.'</h2>'.$MYCODE,$content);
    }}

Answer 1

is_single() is a function returning a boolean value. You could simply use it for your conditional check rather than passing the global variable.

Other than that, your first line of code should be setting $my_global_var with a single equal sign, i.e.: $my_global_var = true;.