Getting the post from start to

Hello !
I’m running a plugin that allows me to execute PHP from posts.
Just trying to call a post inside a page with this plugin.
I want to echo the first part of a post, not the entire post, I created a <!–more–> but still showing the complete post.

Heres the code !

<div id="post1">
<?php
$my_id = 1;
$post_id_1 = get_post($my_id, ARRAY_A);
$post_content = $post_id_1['post_content'];
$post_title = $post_id_1['post_title'];

echo "<strong>\n";
    echo $post_title."\n";
    echo "</strong>";
echo $post_content ;
?>
</div>

I search almost all wordpress.org without a hint.
Please help me !