Writing PHP code in pages without issue?

Question

Writing code in a WP post is not a good idea. As czerspalace mentions the easiest solution is to use a Custom Field to paste the code and use a Shortcode to render it in HTML.

The post would be like:

^{Any number of shortcodes can be used, just put a unique name for each field name,

in this example cod-1 (an arbitrary name).}

The Custom Field:

Plugin code:
_{See Where do I put the code snippets I found here or somewhere else on the web?}

<?php
/**
 * Plugin Name: (SOpt) Shortcode para código HTML
 * Plugin URI: https://pt.stackoverflow.com/a/86941/201
 * Version: 1.0
 * Author: brasofilo 
 */

add_shortcode( 'codigo', 'shortcode_sopt_83496' );

function shortcode_sopt_83496( $atts )
{
    global $post;

    // field not defined, do nothing
    if ( !isset( $atts['field'] ) )
        return '';

    // get custom field attribute value from Shortcode's "field"
    $code = get_post_meta( $post->ID, $atts['field'], true );

    // envelops code in <pre> tag, stilize at will with CSS
    return sprintf(
        '<pre class="codigo-front">%s</pre>', 
        htmlentities( $code, ENT_QUOTES )
    );
}

Result:

_{Any Custom Fields plugin can be used to make a nice interface, instead of WP default.}

^{_{Originally posted at Stack Overflow em Português}}