This is code creates a form so that posts can be created or edited from the front end

gistfile1.txt

<?php
/*
 * Plugin Name: Front Post
 * Plugin URI:  
 * Description: This is a forked code of TheDeadMedic that creates a form so that posts can be created or edited from the front end. See here the original: http://wordpress.stackexchange.com/a/210662/25187
 * Version:     0.1
 * Author:      
 * Author URI:  
 * Text Domain: front-post
 */

class WPSE_Submit_From_Front {
    const NONCE_VALUE = 'frontend_add_edit_post';
    const NONCE_FIELD = 'faep_nonce';

    protected $pluginPath;
    protected $pluginUrl;
    protected $errors = array();
    protected $data = array();

    protected $postID;

    function __construct() {
        $this->pluginPath = plugin_dir_path( __file__ );
        $this->pluginUrl  = plugins_url( '', __file__ );

        if( 'POST' == $_SERVER['REQUEST_METHOD'] && isset( $_POST['postid'] ) ) {
            $this->postID = $_POST['postid']; //echo 'Post ID is ' . $this->postID . '<br>';
        }

        add_action( 'wp_enqueue_scripts', array( $this, 'addStyles' ) );
        add_shortcode( 'post_from_front', array( $this, 'post_shortcode' ) );

        // Listen for the form submit & process before headers output
        add_action( 'template_redirect',  array( $this, 'handleForm' ) );
    }

    function addStyles() {
        wp_enqueue_style( 'submitform-style', "$this->pluginUrl/submit-from-front.css" );
    }

    /**
     * Shortcodes should return data, NOT echo it.
     * 
     * @return string
     */
    function post_shortcode() {
        if ( ! current_user_can( 'publish_posts' ) )
            return sprintf( '<p>To add or edit a post you must be <a href="%s">logged in</a>.</p>', esc_url( wp_login_url(  get_permalink() ) ) );
        elseif ( $this->isFormSuccess() )
            return '<p class="alert-box success"><span>Excellent: </span> The post was saved successfully!</p>';
        else
            return $this->getForm();
    }

    /**
     * Process the form and redirect if sucessful.
     */
    function handleForm() {
        if ( ! $this->isFormSubmitted() )
            return false;

        // http://php.net/manual/en/function.filter-input-array.php
        $data = filter_input_array( INPUT_POST, array(
            'foo_id'           => FILTER_DEFAULT,
            'foo_date'         => FILTER_DEFAULT,
            'item_name'        => FILTER_DEFAULT,
            'item_description' => FILTER_DEFAULT,
            'item_category'    => FILTER_DEFAULT,
            'item_location'    => FILTER_DEFAULT,
            'item_location2'   => FILTER_DEFAULT,
            'contact_person'   => FILTER_DEFAULT,
            'contact_phone'    => FILTER_DEFAULT,
        ));

        $data = wp_unslash( $data );
        $data = array_map( 'trim', $data );

        // You might also want to more aggressively sanitize these fields
        // By default WordPress will handle it pretty well, based on the current user's "unfiltered_html" capability

        $data['item_name']        = sanitize_text_field( $data['item_name'] );
        $data['item_description'] = wp_check_invalid_utf8( $data['item_description'] );
        $data['item_category']    = sanitize_text_field( $data['item_category'] );
        $data['item_location']    = sanitize_text_field( $data['item_location'] );
        $data['item_location2']   = sanitize_text_field( $data['item_location2'] );
        $data['contact_person']   = sanitize_text_field( $data['contact_person'] );
        $data['contact_phone']    = sanitize_text_field( $data['contact_phone'] );

        $this->data = $data;

        if ( ! $this->isNonceValid() )
            $this->errors[] = 'Security check failed, please try again!';

        if ( ! $data['item_name'] )
            $this->errors[] = 'Please add a title!';

        if ( ! $data['item_description'] )
            $this->errors[] = 'Please add a description!';

        if ( ! $data['item_category'] )
            $this->errors[] = 'Please select a category!';

        if ( ! $data['item_location'] )
            $this->errors[] = 'Please select a location!';

        //if( $data['contact_phone'] ) {
            if( ! is_numeric( $data['contact_phone'] ) || $data['contact_phone'] <= 0 || strlen( $data['contact_phone'] ) > 9 ){
                $this->errors[] = 'Add a valid phone number!';
            }
        //}

        if ( ! $this->errors ) {
            $post_id = wp_insert_post( array(
                //'ID'            => $this->postID, // here and only here the $this->postID is empty and I can't use it, so below
                'ID'            => $data['foo_id'], // I used a form hidden input field to get the post ID 
                'post_date'     => $data['foo_date'], // the same for for the post date
                'post_title'    => $data['item_name'],
                'post_content'  => $data['item_description'],
                'post_category' => array( $data['item_category'] ),
                'tax_input'     => array('location' => array( $data['item_location'] )),
                'post_status'   => 'publish',
            ));

            if ( $post_id ) {
                update_post_meta($post_id, 'item_location2', $data['item_location2']);
                update_post_meta($post_id, 'contact_person', $data['contact_person']);
                update_post_meta($post_id, 'contact_phone',  $data['contact_phone']);

                // Redirect to avoid duplicate form submissions
                wp_redirect( add_query_arg( 'success', 'true' ) );
                exit;

            } else {
                $this->errors[] = 'Sorry! Try again!';
            }
        }
    }

    /**
     * Use output buffering to *return* the form HTML, not echo it.
     * 
     * @return string
     */
    function getForm() {

        //if( 'POST' == $_SERVER['REQUEST_METHOD'] && isset( $_POST['postid'] ) ) {
        if( 'POST' == $_SERVER['REQUEST_METHOD'] && !empty( $this->postID ) ) {
			//echo 'Post ID is ' . $this->postID . '<br>'; // here the $this->postID is not empty yet
            $post_to_edit                   = array();
            $post_to_edit                   = get_post( $this->postID );
            $this->data['foo_id']           = $post_to_edit->ID;
            $this->data['foo_date']         = $post_to_edit->post_date;
            $this->data['item_name']        = $post_to_edit->post_title;
            $this->data['item_description'] = $post_to_edit->post_content;
            $category                       = get_the_category( $post_to_edit->ID );
            $this->data['item_category']    = $category[0]->term_id;
            $location                       = get_the_terms( $post_to_edit->ID, 'location' );
            $this->data['item_location']    = $location[0]->term_id;
            $this->data['item_location2']   = get_post_meta( $post_to_edit->ID, 'item_location2', true );
            $this->data['contact_person']   = get_post_meta( $post_to_edit->ID, 'contact_person', true );
            $this->data['contact_phone']    = get_post_meta( $post_to_edit->ID, 'contact_phone',  true );
        }

        ob_start();
        ?>

    <?php foreach ( $this->errors as $error ) : ?>

        <p class="error alert-box"><span>Error: </span><?php echo $error ?></p>

    <?php endforeach ?>

    <form id="frontpostform" method="post"> <!-- autocomplete="off"> -->
        <fieldset>
            <!-- Item name -->
            <label for="item_name">Title<span class="required">*</span></label>
            <input type="text" name="item_name" id="item_name" title="Add a title!" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['item_name'] ) )
                    echo esc_attr( $this->data['item_name'] );

            ?>" />

            <!-- Item description -->
            <label for="item_description">Description<span class="required">*</span></label>
            <textarea name="item_description" id="item_description" rows="10" cols="35" title="Add a description!"><?php

                if ( isset( $this->data['item_description'] ) )
                    echo esc_textarea( $this->data['item_description'] );

            ?></textarea>

            <!-- Category -->
            <label for="item_category">Category<span class="required">*</span></label>
            <select name="item_category" title="Select a category!"> 
                <option value=""><?php echo esc_attr(__('Select ...')); ?></option>
                <?php
                $categories = get_categories( 'hide_empty=0' );
                foreach ($categories as $category) {
					// keep post category value from last POST if there were errors
					if ( isset( $this->data['item_category'] ) && $this->data['item_category'] == $category->cat_ID ) {
  	                   $option = '<option value="' . $category->cat_ID . '" selected="selected">';
  	                } else {
                       $option = '<option value="' . $category->cat_ID . '">';
                    }
	                $option .= $category->cat_name;
	                //$option .= ' (' . $category->category_count . ')';
	                $option .= '</option>';
	                echo $option;
                }
                ?>
            </select>

            <!-- Location -->
            <label for="item_location">Location<span class="required">*</span></label>
            <select name="item_location" title="Select a location!"> 
                <option value=""><?php echo esc_attr(__('Select ...')); ?></option>
                <?php
                $locations = get_categories( 'hide_empty=0&taxonomy=location' );
                foreach ($locations as $location) {
					if ( isset( $this->data['item_location'] ) && $this->data['item_location'] == $location->cat_ID ) {
  	                   $option = '<option value="' . $location->cat_ID . '" selected="selected">';
  	                } else {
                       $option = '<option value="' . $location->cat_ID . '">';
                    }
	                $option .= $location->cat_name;
	                //$option .= ' (' . $location->category_count . ')';
	                $option .= '</option>';
	                echo $option;
                }
                ?>
            </select>

            <!-- Second Location -->
            <div>
            <label for="item_location2">Location 2</label>
            <input type="text" name="item_location2" id="item_location2" title="Location 2 (optional)" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['item_location2'] ) )
                    echo esc_attr( $this->data['item_location2'] );

            ?>" />
            </div>

            <!-- Contact Person -->
            <div>
            <label for="contact_person">Contact person</label>
            <input type="text" name="contact_person" id="contact_person" title="Contact person" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['contact_person'] ) )
                    echo esc_attr( $this->data['contact_person'] );

            ?>" />
            </div>

            <!-- Contact Phone -->
            <div>
            <label for="contact_phone">Contact phone<span class="required">*</span></label>
            <input type="tel" name="contact_phone" id="contact_phone" maxlength="9" title="Contact phone" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['contact_phone'] ) )
                    echo esc_attr( $this->data['contact_phone'] );

            ?>" />
            </div>

            <!-- Submit button -->
			<label for="submitForm"></label>
			<div class="submitForm-wrapper">
			  // below are two hidden fields that will retain the post ID and date
				<input type="hidden" name="foo_id" id="foo_id" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['foo_id'] ) )
                    echo esc_attr( $this->data['foo_id'] );

            ?>" />

				<input type="hidden" name="foo_date" id="foo_date" value="<?php

                // "Sticky" field, will keep value from last POST if there were errors
                if ( isset( $this->data['foo_date'] ) )
                    echo esc_attr( $this->data['foo_date'] );

            ?>" />

                <button type="submit" name="submitForm" id="submitForm" title="Save">Save</button>
            </div>
        </fieldset>

        <?php wp_nonce_field( self::NONCE_VALUE , self::NONCE_FIELD ) ?>
    </form>

        <?php
        return ob_get_clean();
    }

    /**
     * Has the form been submitted?
     * 
     * @return bool
     */
    function isFormSubmitted() {
        return isset( $_POST['submitForm'] );
    }

    /**
     * Has the form been successfully processed?
     *
     * @return bool
     */
    function isFormSuccess() {
        return filter_input( INPUT_GET, 'success' ) === 'true';
    }

    /**
     * Is the nonce field valid?
     * 
     * @return bool
     */
    function isNonceValid() {
        return isset( $_POST[ self::NONCE_FIELD ] ) && wp_verify_nonce( $_POST[ self::NONCE_FIELD ], self::NONCE_VALUE );
    }
}

new WPSE_Submit_From_Front;