I want to add a new field in system.xml
which allows only numeric values. How can I validate that?
Here is my code:
<config>
<sections>
<newsletter translate="label" module="newsletter">
<label>Newsletter</label>
<tab>phone</tab>
<frontend_type>text</frontend_type>
<sort_order>110</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</newsletter>
</sections>
</config>