WordPress Advanced Custom Fields get field key from field name

acf-get-field-key.php

<?php

/**
 * Get field key for field name.
 * Will return first matched acf field key for a give field name.
 * 
 * ACF somehow requires a field key, where a sane developer would prefer a human readable field name.
 * http://www.advancedcustomfields.com/resources/update_field/#field_key-vs%20field_name
 * 
 * This function will return the field_key of a certain field.
 * 
 * @param $field_name String ACF Field name
 * @param $post_id int The post id to check.
 * @return 
 */
function acf_get_field_key( $field_name, $post_id ) {
	global $wpdb;
	$acf_fields = $wpdb->get_results( $wpdb->prepare( "SELECT ID,post_parent,post_name FROM $wpdb->posts WHERE post_excerpt=%s AND post_type=%s" , $field_name , 'acf-field' ) );
	// get all fields with that name.
	switch ( count( $acf_fields ) ) {
		case 0: // no such field
			return false;
		case 1: // just one result. 
			return $acf_fields[0]->post_name;
	}
	// result is ambiguous
	// get IDs of all field groups for this post
	$field_groups_ids = array();
	$field_groups = acf_get_field_groups( array(
		'post_id' => $post_id,
	) );
	foreach ( $field_groups as $field_group )
		$field_groups_ids[] = $field_group['ID'];
	
	// Check if field is part of one of the field groups
	// Return the first one.
	foreach ( $acf_fields as $acf_field ) {
		if ( in_array($acf_field->post_parent,$field_groups_ids) )
			return $acf_field->post_name;
	}
	return false;
}

@Hollyw00d Allow me. There are few ways:

1. Use get field object - if you have the exact selector, its simple
2. Parse the acf-json/*.json and extract keys from there. It can done on the fly Or you may generate your own index
   Hope that helps

@Mulli I used your suggestion below to add the ACF field ID (like field_63c84b9cb5f66):

Parse the acf-json/*.json and extract keys from there. It can done on the fly Or you may generate your own index

I can't display the code here as I used it for my job but this another option to get the ACF field ID without making database calls.

Or one can simply use acf_get_reference()