Call to undefined function update_post_meta() - php

Question

I am getting this error:
Fatal error: Call to undefined function update_post_meta()
My code is as below. Can any one help me out?
<?php if(isset($_POST['submit']))
{
$metavalue = $_POST['usertext'];
$postid = $_POST['postid'];
$metakey = 'my_key';
update_post_meta( $postid, $metakey, $metavalue );
} ?>
<div class="wrap">
<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="hidden" name="postid" value="<?php echo $_GET['value'] ?>" >
<input type="hidden" name="123" value="123" >
<input type="text" name="usertext">
<br/>
<input type="submit" name="submit" value="Update">
</form>
</div>
<?php

Answer 1

I'm guessing this is within a theme of some sort. You need to assign a post to the global $post variable.
Try
//your stuff until here
$metakey = 'my_key';
$current_post = get_post($_POST['postid']);
if($current_post){
global $post = $current_post
setup_postdata($post);
update_post_meta( //continue
//more continueing
} else {
//throw some error
}
get_post gets the post based on some sort of variable, or if you give it nothing it returns a false.
global $post is the WP global variable to hold your current post. Be careful using this as it might mess up code after this if you're assuming the loop continues automatically, or if you need the loop.
setup_postdata Sets up global post data. Helps to format custom query results for using Template tags.
Hopefully this will help you out. Next time you ask a question also have a look at this and this.