<?php

$source=<<<EOT
<select class="parent1">
    <option value="1">First child</option>
    <option value="2">Second child</option>
    <option value="3">Third child</option>
</select>

<select class="parent2">
    <option value="4">Fourth child</option>
    <option value="5">Fifth child</option>
    <option value="6">Sixth child</option>
</select>
EOT;

preg_match_all(
    '~(?:<select class="parent1">|\G)\s*\K<option value="(.*?)">(.*?)</option>~',
    $source, $matches);
    
print_r($matches);
?>