- array_search: documentation ( source)
- array_rand: documentation ( source)
- json_encode: documentation ( source)
<?php
$lines = array('link1', 'link2', 'link3');
$imgSrc = $lines[array_rand($lines)];
$lineNumber = array_search($imgSrc, $lines);
echo json_encode(array($lineNumber => $imgSrc));