如何在另一个函数中执行php函数？

I want to show a css button and text field in PHP(functions.php of WordPress theme).

I am able to do everything but the issue is just the value="<?php echo wp_get_shortlink(get_the_ID()); ?>" which does't execute if I use double quotes.

Here is my code:

function add_buttons_filter( $content ) {


      $content .= ' <span class="abf">
                    <span class="abc">
                        <button class="bcd"><span class="dcb"><span></span></span> 
                        </button>
                    </span>
                    <span class="abf">
                            <span class="abc-main">
                            <span class="abc-box" style="display:none;"> 
                                <input type="text" style=" max-width: 80px;" value="<?php echo wp_get_shortlink(get_the_ID()); ?>" onclick="this.focus(); this.select();" />
                            </span>
                            </span>
                    </span>
                    </span>';    
return $content;

}
add_filter( 'the_content', 'add_buttons_filter' );

Could any one help me or show me a better solution to execute this in functions.php ?

Your statement is already embedded in php-Code, so it won't help to insert further code there - you can directly do the stuff:

function add_buttons_filter( $content ) {


      $content .= ' <span class="abf">
                    <span class="abc">
                        <button class="bcd"><span class="dcb"><span></span></span> 
                        </button>
                    </span>
                    <span class="abf">
                            <span class="abc-main">
                            <span class="abc-box" style="display:none;"> 
                                <input type="text" style=" max-width: 80px;" value="' .  wp_get_shortlink(get_the_ID()) . '" onclick="this.focus(); this.select();" />
                            </span>
                            </span>
                    </span>
                    </span>';    
return $content;

}
add_filter( 'the_content', 'add_buttons_filter' );

Try like this

<input type="text" style=" max-width: 80px;" value='.wp_get_shortlink(get_the_ID()).' onclick="this.focus(); this.select();" />

use this

value="' . wp_get_shortlink(get_the_ID()) . '"