A FAPI element using a textarea with xml validation.
<?php
$form['xml_config'] = array(
'#type' => 'xmltext',
'#title' => 'XML config',
'#default_value' => variable_get('xml_config', ''),
'#xml_schema' => '<xs:schema>...</xs:schema>',
'#xml_example' => '<something>...</something>',
);
Enable xml_field_codemirror module to introduce this FAPI element, which creates a CodeMirror input field with xml validation.
Refer to the online API for more info.
You may alter the global default options using hook_xml_field_codemirror_config_alter(). For fields you may alter some configuration options in the field UI. To control the CodeMirror options at the element level add/alter the key #codemirror_options
:
<?php
$form['xml_config'] = array(
'#type' => 'xml_codemirror',
'#title' => 'XML config',
'#default_value' => variable_get('xml_config', ''),
'#codemirror_options' => array(
'tabSize' => 2,
);
);