WordPress Reserved Global Variables, You Should Avoid Define A Variable Using These Name

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Global variable is a variable that is accessible in every scope, in PHP it works ONLY for the same page and the file that are included after. However, some predefined variables, known as superglobals are always accessible in whole site. Both of global and superglobal variable can be redefined or overwrite it’s value.

When developing a WordPress Plugin and or Theme, sometimes you use the PHP global variable. That’s okay, but it’s HIGHLY RECOMMENDED that you not use variable name that are already defined by WordPress. Why?  Because it may break your other code that may intended to use WordPress variable.

Example

Example case:

1. You have this code somewhere in your theme:

$cat = 5; /*define cat global variable */
$paged = 2; /*define paged global variable */

/* Those variable are intended to this custom query */

query_posts("cat=$cat&posts_per_page=10&paged=$paged");
if (have_posts()) while (have_posts()) : the_post();
	echo get_the_title().'<br />';
endwhile;

The result is you will get the list of 10 post title from category 5, paged 2. That way as what you expected.

2. On the other hand, you (or other developer) also have this code:

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query_posts("cat=$cat&posts_per_page=10&paged=$paged");
if (have_posts()) while (have_posts()) : the_post();
    echo get_the_title().'<br />';
endwhile;

query_posts("cat=$cat&posts_per_page=10&paged=$paged");
if (have_posts()) while (have_posts()) : the_post();
	echo get_the_title().'<br />';
endwhile;

Here, you are not define $cat and $paged variable. For the reason that your purpose is to get post in current category ($cat) and current page ($paged) since $cat and $paged variable is always available as global variable in WordPress depending on which page you’re accessed. But what happened is unexpected, because those global variable was already previously redefined (in example #1).

So, do not redefine a reserved WordPress global variable unless you are know what are you doing.

The lists of WordPress Reserved Variable

Anyway, what is the variables that reserved WordPress? Here is the list, the left side is variable name, and the right side is the type:

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$_template_file = string
$require_once = boolean
$posts = array
$post = object
$wp_did_header = boolean
$wp_did_template_redirect = NULL
$wp_query = object
$wp_rewrite = object
$wpdb = object
$wp_version = string
$wp = object
$id = integer
$comment = NULL
$user_ID = integer
$cat = string
$paged = integer
$error = string
$m = integer
$p = integer
$post_parent = string
$subpost = string
$subpost_id = string
$attachment = string
$attachment_id = integer
$name = string
$static = string
$pagename = string
$page_id = integer
$second = string
$minute = string
$hour = string
$day = integer
$monthnum = integer
$year = integer
$w = integer
$category_name = string
$tag = string
$tag_id = string
$author_name = string
$feed = string
$tb = string
$comments_popup = string
$meta_key = string
$meta_value = string
$preview = string
$s = string
$sentence = string
$fields = string
$category__in = array
$category__not_in = array
$category__and = array
$post__in = array
$post__not_in = array
$tag__in = array
$tag__not_in = array
$tag__and = array
$tag_slug__in = array
$tag_slug__and = array
$ignore_sticky_posts = boolean
$suppress_filters = boolean
$cache_results = boolean
$update_post_term_cache = boolean
$update_post_meta_cache = boolean
$post_type = string
$posts_per_page = integer
$nopaging = boolean
$comments_per_page = string
$no_found_rows = boolean
$order = string

$_template_file = string
$require_once = boolean
$posts = array
$post = object
$wp_did_header = boolean
$wp_did_template_redirect = NULL
$wp_query = object
$wp_rewrite = object
$wpdb = object
$wp_version = string
$wp = object
$id = integer
$comment = NULL
$user_ID = integer
$cat = string
$paged = integer
$error = string
$m = integer
$p = integer
$post_parent = string
$subpost = string
$subpost_id = string
$attachment = string
$attachment_id = integer
$name = string
$static = string
$pagename = string
$page_id = integer
$second = string
$minute = string
$hour = string
$day = integer
$monthnum = integer
$year = integer
$w = integer
$category_name = string
$tag = string
$tag_id = string
$author_name = string
$feed = string
$tb = string
$comments_popup = string
$meta_key = string
$meta_value = string
$preview = string
$s = string
$sentence = string
$fields = string
$category__in = array
$category__not_in = array
$category__and = array
$post__in = array
$post__not_in = array
$tag__in = array
$tag__not_in = array
$tag__and = array
$tag_slug__in = array
$tag_slug__and = array
$ignore_sticky_posts = boolean
$suppress_filters = boolean
$cache_results = boolean
$update_post_term_cache = boolean
$update_post_meta_cache = boolean
$post_type = string
$posts_per_page = integer
$nopaging = boolean
$comments_per_page = string
$no_found_rows = boolean
$order = string