Wordpress show only on posts - php

Question

i have a problem with my WP function. I need it to ONLY be shown on blog posts. Right now it get shown on the other pages such as the frontpage.
Here is my function in functions.php:
function my_sp_function( $content ) {
$include = FALSE;
$post = FALSE;
if ( in_category( array( 3, 4, 5, 6, 91, 133 ) ) ) {
$include = TRUE;
}
if (get_post_type(get_the_ID()) == 'post'){
$post = TRUE;
}
if ( $post = TRUE AND $include == TRUE ) {
return $content;
}
}
Anybody there knows what i do wrong?
I have tried with this code now, it still shows the $content on the front page..
function my_sp_function( $content ) {
$include = FALSE;
$post = FALSE;
if ( in_category( array( 3, 4, 5, 6, 91, 133 ) ) ) {
$include = TRUE;
}
if( is_home() || is_front_page() ) {
$include = FALSE;
}
if ( $post == TRUE AND $include == TRUE ) {
return $content;
}
}

Answer 1

If you would like to include your function only on the home page you have two options - only insert the function into the single.php template in your theme, or the prefered method is to only hook your function to the_content when is_single() is true but is_page() is false.
function my_sp_function( $content ) {
$content .= "Some more stuff for the_content!";
return $content;
}
if ( is_single() && ! is_page() ){
add_filter( 'the_content', 'my_sp_function' );
}