Answers for "run a shortcode in function.php"

<?php echo do_shortcode(' '); ?> with variable

$childrens = do_shortcode( '[products ids="'. $string . '"]' );

how to create shortcode with php

// function that runs when shortcode is called
function wpb_demo_shortcode() { 
 
// Things that you want to do. 
$message = 'Hello world!'; 
 
// Output needs to be return
return $message;
} 
// register shortcode
add_shortcode('greeting', 'wpb_demo_shortcode');