category-meta-field-add-form.php

<?php
/**
 * Category Meta Fields (add form).
 * @param string $taxonomy Taxonomy slug.
 */
function post_category_add_extra_fields( $taxonomy ) { ?>

    <div class="form-field extra-input-wrap">
        <label for="text"><?php _e( 'Extra Input Field', 'textdomain' ); ?></label>
        <input type="text" name="extra_input" id="extra-input" size="40">
        <p><?php _e( 'Extra input field description', 'textdomain' ); ?></p>
    </div>

<?php
}
add_action( 'category_add_form_fields', 'post_category_add_extra_fields' );

category-meta-field-edit-form-furnished.php

<?php
/**
 * Category Meta Fields (edit form).
 * @param  object $taxonomy Taxonomy object.
 */
function post_category_edit_extra_fields( $taxonomy ) {
	$saved_meta = get_option( "tax_meta{$taxonomy->term_id}" );
	//var_dump($saved_meta); //for debugging
	?>

    <tr class="form-field">
        <th valign="top" scope="row">
            <label for="extra-input"><?php _e( 'Extra Input Field', 'textdomain' ); ?></label>
        </th>
        <td>
            <input type="text" name="extra_input" id="extra-input" value="<?php if( !empty($saved_meta['extra_input']) ) echo $saved_meta['extra_input']; ?>" size="40">
            <p class="description"><?php _e( 'Extra input field description', 'textdomain' ); ?></p>
        </td>
    </tr>

<?php
}
add_action( 'category_edit_form_fields', 'post_category_edit_extra_fields' );

category-meta-field-edit-form.php

<?php
/**
 * Category Meta Fields (edit form).
 * @param  object $taxonomy Taxonomy object.
 */
function post_category_edit_extra_fields( $taxonomy ) { ?>

    <tr class="form-field">
        <th valign="top" scope="row">
            <label for="extra-input"><?php _e( 'Extra Input Field', 'textdomain' ); ?></label>
        </th>
        <td>
            <input type="text" name="extra_input" id="extra-input" value="" size="40">
            <p class="description"><?php _e( 'Extra input field description', 'textdomain' ); ?></p>
        </td>
    </tr>

<?php
}
add_action( 'category_edit_form_fields', 'post_category_edit_extra_fields' );

category-meta-field-save-data.php

<?php
/**
 * Saving Category Meta Fields.
 * @param  integer $term_id  Term ID.
 * @param  integer $tt_id    Term Taxonomy ID.
 * @param  string  $taxonomy Taxonomy slug.
 */
function post_category_save_extra_fields( $term_id, $tt_id, $taxonomy ) {

    //Handle the Category only
    if( $taxonomy === 'category' ) {

        $category_meta_array = array();

        if( isset($_POST['extra_input']) )
            $category_meta_array['extra_input'] = sanitize_text_field( $_POST['extra_input'] );

        update_option( "tax_meta{$term_id}", $category_meta_array );
    }
}
add_action( 'edit_term', 'post_category_save_extra_fields', 10, 3 );
add_action( 'create_term', 'post_category_save_extra_fields', 10, 3 );

functions.php

<?php
/**
 * Category Meta Fields (add form).
 * @param string $taxonomy Taxonomy slug.
 */
function post_category_add_extra_fields( $taxonomy ) { ?>

    <div class="form-field extra-input-wrap">
        <label for="text"><?php _e( 'Extra Input Field', 'textdomain' ); ?></label>
        <input type="text" name="extra_input" id="extra-input" size="40">
        <p><?php _e( 'Extra input field description', 'textdomain' ); ?></p>
    </div>

<?php
}
add_action( 'category_add_form_fields', 'post_category_add_extra_fields' );


/**
 * Category Meta Fields (edit form).
 * @param  object $taxonomy Taxonomy object.
 */
function post_category_edit_extra_fields( $taxonomy ) {
	$saved_meta = get_option( "tax_meta{$taxonomy->term_id}" );
	//var_dump($saved_meta); //for debugging
	?>

    <tr class="form-field">
        <th valign="top" scope="row">
            <label for="extra-input"><?php _e( 'Extra Input Field', 'textdomain' ); ?></label>
        </th>
        <td>
            <input type="text" name="extra_input" id="extra-input" value="<?php if( !empty($saved_meta['extra_input']) ) echo $saved_meta['extra_input']; ?>" size="40">
            <p class="description"><?php _e( 'Extra input field description', 'textdomain' ); ?></p>
        </td>
    </tr>

<?php
}
add_action( 'category_edit_form_fields', 'post_category_edit_extra_fields' );


/**
 * Saving Category Meta Fields.
 * @param  integer $term_id  Term ID.
 * @param  integer $tt_id    Term Taxonomy ID.
 * @param  string  $taxonomy Taxonomy slug.
 */
function post_category_save_extra_fields( $term_id, $tt_id, $taxonomy ) {

    //Handle the Category only
    if( $taxonomy === 'category' ) {

        $category_meta_array = array();

        if( isset($_POST['extra_input']) )
            $category_meta_array['extra_input'] = sanitize_text_field( $_POST['extra_input'] );

        update_option( "tax_meta{$term_id}", $category_meta_array );
    }
}
add_action( 'edit_term', 'post_category_save_extra_fields', 10, 3 );
add_action( 'create_term', 'post_category_save_extra_fields', 10, 3 );

hooks-at-a-glance.php

<?php
function function_name_here() {
  //...
}
add_action( 'category_add_form_fields', 'function_name_here' ); //hooked for default term category (Category)
add_action( 'post_tag_add_form_fields', 'function_name_here' ); //hooked for default term 'pst_tag' (Tags)
add_action( 'taxonomy_one_add_form_fields', 'function_name_here' ); //hooked for custom taxonomy 'taxonomy_one'
add_action( 'taxonomy_two_add_form_fields', 'function_name_here' ); //hooked for custom taxonomy 'taxonomy_two'

how-serialize-works.php

<?php
//Let's have an array
$_array = array( 'extra_input' => 'data I stored', 'file_url' => 'http://localhost/project/wp-content/uploads/2015/09/image.jpg' );
var_dump( $_array ); //how our array looks like

//Let's serialize it
$_serialized = serialize( $_array );
var_dump( $_serialized ); //what our serialization produced

necessary-functions.php

<?php
/**
 * Get Taxonomy Meta Field.
 * @param  mixed  $term_id  Term ID or Term Object.
 * @param  string $meta_key Meta Key to retrieve its data.
 * @return mixed            Meta value as string or as array.
 */
function nano_get_tax_meta( $term_id, $meta_key = false ){
    $_term_id = ( is_object( $term_id ) ) ? $term_id->term_id : $term_id;
    $saved_meta = get_option( "tax_meta{$_term_id}" );

    if( $meta_key ) {
	    if( isset( $saved_meta[$meta_key] ) ){
	      return $saved_meta[$meta_key];
	    } else {
	      return '';
	    }
	} else {
		return $saved_meta;
	}
}


/**
 * Get taxonomy meta data.
 * @param  mixed $term_id   Term ID or Term Object.
 * @param  string $meta_key Meta Key to retrieve its data.
 * @param  string $size     Image size: medium, thumbnail, large, full or custom size.
 * @return string           Image URL only.
 */
function nano_get_tax_meta_img_src( $term_id, $meta_key, $size = 'thumbnail' ) {
	$img_meta = nano_get_tax_meta( $term_id, $meta_key );

	if( !empty($img_meta) )
		$img_url = wp_get_attachment_image_src( absint( $img_meta ), $size );

	if( $img_url )
		return $img_url[0];
	else
		return '';
}


/**
 * Delete Taxonomy Meta.
 * @param  integer|object  $term_id  Term ID or Term Object.
 * @param  string $meta_key Default is false, or the meta key to delete only.
 */
function nano_delete_tax_meta( $term_id, $meta_key = false ) {
	$_term_id = ( is_object( $term_id ) ) ? $term_id->term_id : $term_id;
	$term_meta = nano_get_tax_meta( $_term_id, $meta_key );

	if( $meta_key ) {
	    if( isset( $term_meta[$meta_key] ) )
	    	unset( $term_meta[$meta_key] );	
    	
    	update_option( "tax_meta{$_term_id}", $term_meta );
	} else {
		delete_option( "tax_meta{$_term_id}" );
	}
}


/**
 * Add/Update Taxonomy Meta.
 * @param  integer $term_id|object    Term ID or Term Object.
 * @param  string $meta_key   Taxonomy Meta Key to update its value.
 * @param  string $meta_value New value to update with.
 */
function nano_update_tax_meta( $term_id, $meta_key, $meta_value ) {
	$_term_id = ( is_object( $term_id ) ) ? $term_id->term_id : $term_id;
	$term_meta = nano_get_tax_meta( $_term_id, $meta_key );

	if( isset( $term_meta[$meta_key] ) ) {
		$term_meta[$meta_key] = $meta_value;
		update_option( "tax_meta{$_term_id}", $term_meta );
	}
}

saving-image-in-options-table.php

<?php
/**
 * Get image ID from full URL.
 * @link https://pippinsplugins.com/retrieve-attachment-id-from-image-url/
 * @link http://codex.wordpress.org/Function_Reference/url_to_postid
 * 
 * @param  string $image_url Full URL of an image.
 * @return integer           Attachment ID of the image.
 */
function nanodesigns_get_image_id( $image_url ) {
	global $wpdb;
	if( $image_url ) {
		$attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
		return $attachment[0];
	} else {
		return '';
	}
}

/**
 * Saving Category Meta Fields.
 * @param  integer $term_id  Term ID.
 * @param  integer $tt_id    Term Taxonomy ID.
 * @param  string  $taxonomy Taxonomy slug.
 */
function post_category_save_image_field( $term_id, $tt_id, $taxonomy ) {

    //Handle the Category only
    if( $taxonomy === 'category' ) {

        $category_meta_array = array();

        if( isset($_POST['image_url']) ) {
        	$img_id = nanodesigns_get_image_id( sanitize_text_field( $_POST['image_url'] ) );
          	$category_meta_array['image_id'] = absint( $img_id );
        }

        update_option( "tax_meta{$term_id}", $category_meta_array );
    }
}
add_action( 'edit_term', 'post_category_save_image_field', 10, 3 );
add_action( 'create_term', 'post_category_save_image_field', 10, 3 );