What are Important Drupal Functions in common.inc?

1. base_path()
Syntax:
base_path()

Description:
Returns the base URL path of the Drupal installation. At the very least, this will always default to.

2. drupal_add_css()
Syntax:
drupal_add_css($path = NULL, $type = 'module', $media = 'all', $preprocess = TRUE)

Description:
Adds a CSS file to the stylesheet queue.

Example:
drupal_add_css(drupal_get_path('module', 'book') .'/book.css');

3. drupal_add_js()
Syntax:
drupal_add_js($data = NULL, $type = 'module', $scope = 'header', $defer = FALSE, $cache = TRUE, $preprocess = TRUE)

Description:
Add a JavaScript file, setting or inline code to the page.

Example:
drupal_add_js(drupal_get_path('module', 'gems_common') . '/common.js');

4. drupal_error_handler()
Syntax:
drupal_error_handler($errno, $message, $filename, $line, $context)

Description:
 Log errors as defined by administrator.

 Error levels:
 - 0 = Log errors to database.
 - 1 = Log errors to database and to screen.

5. drupal_eval()
Syntax:
drupal_eval($code)

Description:
Evaluate a string of PHP code.  A string containing the printed output of the code, followed
by the returned output of the code.

6. drupal_get_breadcrumb()
Syntax:
drupal_get_breadcrumb()

Description:
Get the breadcrumb trail for the current page.

7. drupal_get_content()
Syntax:
drupal_get_content($region = NULL, $delimiter = ' ')

Description:
Get assigned content in all or specific region.

8. drupal_get_html_head()
Syntax:
drupal_get_html_head()

Description:
Retrieve output to be displayed in the head tag of the HTML page.

9. drupal_goto()
Syntax:
drupal_goto($path = '', $query = NULL, $fragment = NULL, $http_response_code = 302)

Description:
Send the user to a different Drupal page.

10. drupal_json()
Syntax:
drupal_json($var = NULL)

Description:
Return data in JSON format.

11. drupal_not_found()
Syntax:
drupal_not_found()

Description:
Generates a 404 error if the request can not be handled.

12. drupal_access_denied()
Syntax:
drupal_access_denied()

Description:
Generates a 403 error if the request is not allowed.

13. drupal_render()
Syntax:
drupal_render(&$elements)


Description:
Renders HTML given a structured array tree.

14. drupal_site_offline()
Syntax:
drupal_site_offline()

Description:
Generates a site off-line message.

15. format_date()
Syntax:
format_date($timestamp, $type = 'medium', $format = '', $timezone = NULL, $langcode = NULL)

Description:
Format a date with the given configured format or a custom format string.

16. l()
Syntax:
4.6 – 5 l($text, $path,
$attributes
= array(), $query = NULL, $fragment = NULL, $absolute = FALSE, $html = FALSE) 6 l($text, $path,
$options
= array()) 7 – 8 l($text, $path, array $options = array())
Description:
Format an internal Drupal link.

Example #1: using Drupal 4-5
l(
  'An <strong>awesome</strong> link!',
  'node/56',
  array('class' => 'widelink'),
  NULL,
  NULL,
  FALSE,
  TRUE);

 
Example #2: using Drupal 6

print l('Privacy Policy', 'privacypolicy', array('attributes' => array('target' => '_blank')));

17. t()
Syntax:
t($string, $args = array(), $langcode = NULL)


Description:
Translate strings to the page language or a given language.

Example:
$output .= '<p>'. t('Go to the <a href="@contact-page">contact page</a>.', array('@contact-page' => url('contact'))) .'</p>';

18. valid_email_address()
Syntax:
valid_email_address($mail)

Description:
Verify the syntax of the given e-mail address.

Return:  TRUE if the address is in a valid format.

19. valid_url()
Syntax:
valid_url($url, $absolute = FALSE)

Description:
Verify the syntax of the given URL.

Return: TRUE if the URL is in a valid format.