function replace_content( $content ) {
if( is_single() ) {
$search = array( $content );

$feat_image = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) );
$url = "https://www.site.com/video.php?x12345=" . wp_strip_all_tags( get_the_conte nt());

        $url = str_replace( "[player1]", "", $url );
        $url = str_replace( "[/player1]", "", $url );
        
        
        $replace = array( '<video class="video-js vjs-default-skin" controls preload="auto" width="100%" height="450" poster="' . $feat_image . '" data-setup="{}"><source src="' . $url . '" type='video/mp4'></video>' );
        
        $content = str_replace( $search, $replace, $content );
        return $content;
    }
}

add_filter( 'the_content', 'replace_content' );