<?php

$numeros = array('one' => 1, 'two' => 2, 'three' => 3, 'my_one' => 55, 'my_two' => 33);

$myNumeros = array_filter(array_keys($numeros), function ($chave){
	return (strpos($chave, 'my_') !== false);
	});

print_r($myNumeros);