<?php
$a = array();
 
$a['Post1'] = array();
$a['Post2'] = array();
$a['Post3'] = array();
 
$a['Post1']['id'] = 1;
$a['Post1']['title'] = 'hi';
 
$a['Post2'] = array(
    array('user_id' => 1),
    array('user_id' => 2)
);
 
$a['Post3'] = array(
    array('user_name' => 1)
);
 
function printValuesByKey($array, $key) {
    if (!is_array($array)) return;
    if (isset($array[$key])) 
        echo $key .': '. $array[$key] .'<br>';
    else
        foreach ($array as $v)
            printValuesByKey($v, $key);
}
 
printValuesByKey($a, 'user_id');

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