Custom module for form I have created form by implementing custom module. Can anybody please guide about writing submit function for my form.

function custom_form_module_form($form,&amp;$form_state) {
  $form['name'] = array(
   '#title' =&gt; 'NAME',
   '#type' =&gt; 'textfield',
  );
  $form['company'] = array(
    '#title' =&gt; 'COMPANY',
    '#type' =&gt; 'textfield',
  );
  $form['phone'] = array(
    '#title' =&gt; 'PHONE',
    '#type' =&gt; 'textfield',
  );
  $form['email'] = array(
    '#title' =&gt; 'EMAIL',
    '#type' =&gt; 'textfield',
  );
  $form['message'] = array(
    '#title' =&gt; 'MESSAGE',
    '#type' =&gt; 'textarea',
  );
  $form['submit_button']=array(
    '#type'=&gt;'submit',
    '#value'=&gt;t('SEND'),
  );
  return $form;
}


UPDATED 

 function custom_form_module_form_validate($form, &amp;$form_state)
{
 if((!isset($_GET[($form_state['values']['name'])]))||
 (!isset($_GET[($form_state['values']['company'])]))||
 (!isset($_GET[($form_state['values']['phone'])]))||
 (!isset($_GET[($form_state['values']['email'])]))||
 (!isset($_GET[($form_state['values']['message'])])))
 {
  form_set_error(t('Please fill the entire form'));
 }
 $valid_email = $form_state['values']['email'];
 if (!valid_email_address($valid_email))
 {
  form_set_error('email', 'Sorry. Your email address,' . $valid_email . ', is not valid. Please submit a valid E-mail address.');
 }
 if(!($form_state['values']['company'] &gt; 1))
 {
  form_set_error('company', t('Company does not appear to be valid.'));
 }
 if (!($form_state['values']['message'] &gt; 2 ))
 {
  form_set_error('message', t('message does not appear to be valid.'));
 }
}


and below is my submit function. Is this right? because I am not receiving email as I specified my own email address.

function custom_form_module_form_mail($key, &amp;$message, $params)
{
 $headers = array(
 'MIME-Version' =&gt; '1.0',
 'Content-Type' =&gt; 'text/html; charset=UTF-8;',
 'Content-Transfer-Encoding' =&gt; '8Bit',
 'X-Mailer' =&gt; 'Drupal'
  );
  foreach ($headers as $key =&gt; $value)
  {
   $message['headers'][$key] = $value;
  }
  $message['subject'] = $params['subject'];
  $message['body'] = $params['body'];
}

 function custom_form_module_form_submit($form, &amp;$form_state) 
{
  $valid_email = $form_state['values']['email'];
  $body[] = 'Email: '.$valid_email.'&lt;br /&gt;URL: '.request_uri();
  $to = 'me@example.com';
  $params = array(
  'body' =&gt; $body,
  'subject' =&gt; 'Welcome!',
  );
  if (drupal_mail('custom_form_module_form', 'some_mail_key', $to, language_default(), $params, TRUE))
  {
    drupal_set_message('Thanks, we will be in contact with more information soon.');     
  } 
  else
  {
    drupal_set_message('There was an error subscribing you. Please try again later');
  }
}
You can add '#submit' =&gt; array('SOME_FUNCTION') to your submit button and write your function normally.