Get a WordPress post with a specific value in the custom fields

get_post_by_custom_field.php

<?php

function get_post_by_custom_field($args)
{
    $defaults = array(
        'post_type' => 'post',
        'key'       => '',
        'value'     => '',
        'compare'   => '='
    );

    $args = wp_parse_args($args, $defaults);

    $search = new WP_Query(array(
        'post_type'  => $args['post_type'],
        'meta_query' => array(
            array(
                'key'     => $args['key'],
                'value'   => $args['value'],
                'compare' => $args['compare']
            )
        )
    ));

    // found the post ?
    if ($search->have_posts()) {
        $search->the_post();

        // return the post
        return $post;
    }

    return false;
}

// Example
$args = array(
    'key'     => 'uniqueid',
    'value'   => $uid,
    'compare' => '='
);

$post = get_post_by_custom_field($args);

// No post found ?
if (!$post) {
}