Lock/unlock a field
$field_name = 'body';
$field = field_read_field($field_name);
$field['locked'] = 0; // 0: unlock; 1: lock.
field_update_field($field);
$field_name = 'body';
$field = field_read_field($field_name);
$field['locked'] = 0; // 0: unlock; 1: lock.
field_update_field($field);