Check and add to the post content - php

Question

How to check if post have meta data and display it by function?
I have post type Portfolio and meta fields (_wi_next-script-part and _wi_prev-script-part)
function wi_next_prev_part_links($content) {
global $wp_query;
$postid = $wp_query->post->ID;
if ( is_singular( 'portfolio' ) ) {
if ( get_post_meta( $postid, '_wi_prev-script-part', true ) ) {
$prev_script_part = get_post_meta( $postid, '_wi_prev-script-part', true );
$content .= '← Prev Part Link';;
}
if ( get_post_meta( $postid, '_wi_next-script-part', true ) ) {
$next_script_part = get_post_meta( $postid, '_wi_next-script-part', true );
$content .= 'Next Part Link →';
}
}
return $content;
}
add_filter ('the_content', 'wi_next_prev_part_links', 0);

Answer 1

There are two ways of doing this. One is to check the value of the meta key and one is to check if the meta key exists.
Checking the value
As explained in the get_post_meta documentation the function will return an empty string or an empty array if the meta key your tried accessing does not exists. Since both an empty string and an empty array are falsy you can use a simple if statement to check whether a meta key has valid meta data or not as such:
if ( get_post_meta( $postid, '_wi_prev-script-part', true ) )
{
//Do something
}
Checking for the key
However, in some instances an empty string is considered to be a valid value. If your code regards empty string as a desirable value you will need to check the for the existence of the meta key itself. We can do that with the get_post_custom_keys function:
if( in_array( '_wi_prev-script-part', get_post_custom_keys($postid) ) )
{
//Do something
}