Save custom select options

acf_save_custom_select_options.php

<?php
/**
 * Save custom select options
 *
 **/
function acf_save_custom_select_options( $value, $post_id, $field ) {
	if ( ! $field['allow_custom'] || ! $field['{theme_name}_save_custom'] ) {
		return $value;
	}

	// Force value to array
	$_value = acf_array( $value );

	// Vars
	$values  = array();
	$choices = $field['choices'];

	// Updater
	$update = false;

	// Construct Choices
	foreach ( $_value as $v ) {

		// Explode value
		$_v = explode( ' : ', $v );

		$values[] = $_v[0];

		// value : text wasn't found
		if ( ! isset( $_v[1] ) ) {
			$_v[1] = $v;
		}

		// ignore if already exists
		if ( isset( $choices[ $_v[0] ] ) ) {
			continue;
		}

		$update = true;

		// unslash & sanitize (remove tags) value
		$_v[0] = wp_unslash( $_v[0] );
		$_v[0] = sanitize_text_field( $_v[0] );

		// unslash & sanitize (remove tags) text
		$_v[1] = wp_unslash( $_v[1] );
		$_v[1] = sanitize_text_field( $_v[1] );

		// append
		$choices[ $_v[0] ] = $_v[1];

	}

	if ( $update ) {

		// Inspired by Checkbox field
		$selector = $field['ID'] ? $field['ID'] : $field['key'];
		$field    = acf_get_field( $selector );

		// bail early if no ID (JSON won't be updated)
		if ( $field['ID'] ) {

			$field['choices'] = $choices;

			// Save field choices
			acf_update_field( $field );

		}
	}

	if ( is_array( $value ) ) {
		return $values;
	}

	return $values[0];
}
add_filter( 'acf/update_value/type=select', 'acf_save_custom_select_options', 10, 3 );

/**
 * Add "Save custom" option to field settings
 * The main save_custom doesn't work
 *
 **/
function acf_add_save_custom_select_option( $field ) {
	acf_render_field_setting(
		$field,
		array(
			'label'      => __( "Save 'custom' values to the field's choices", 'acf' ),
			'name'       => '{theme_name}_save_custom',
			'type'       => 'true_false',
			'ui'         => 1,
			'message'    => __( 'Save Custom', 'acf' ),
			'conditions' => array(
				'field'    => 'allow_custom',
				'operator' => '==',
				'value'    => 1,
			),
		)
	);
}
add_filter( 'acf/render_field_settings/type=select', 'acf_add_save_custom_select_option' );