* mypluck.php

<?php
// array_column(PHP 5>=5.5.0, PHP 7)
// http://php.net/manual/en/function.array-column.php

function myarraymap(callable $f, array $a) : array {
    $ret = [];    
    for ($i = 0, $n = count($a); $i < $n; $i++) {
        $ret[$i] = $f($a[$i], $i);
    }
    return $ret;
}

function mypluck(array $a, string $col) {
    // (PHP 4 >= 4.0.6, PHP 5, PHP 7)
    // array_map — Applies the callback to the elements of the given arrays
    return myarraymap(function($item) use ($col) {
        return  $item[$col];
    }, $a);
}

// Array representing a possible record set returned from a database
$records = array(
    array(
        'id' => 2135,
        'first_name' => 'John',
        'last_name' => 'Doe',
    ),
    array(
        'id' => 3245,
        'first_name' => 'Sally',
        'last_name' => 'Smith',
    ),
    array(
        'id' => 5342,
        'first_name' => 'Jane',
        'last_name' => 'Jones',
    ),
    array(
        'id' => 5623,
        'first_name' => 'Peter',
        'last_name' => 'Doe',
    )
);

$first_names = mypluck($records, 'first_name');
print_r($first_names);
 

* test

$ php mypluck.php Array (     [0] => John     [1] => Sally     [2] => Jane     [3] => Peter )