<?php

/* Takes the_content as input and does a string replace to convert
standard img tags into amp-img tags with an optional layout argument.
If no layout type is supplied it defaults to responsive. */
function ampImageConverter( $html, $layout = 'responsive' ) {

    // Preserve formatting from the post content
    $html = wpautop($html, true);

    // Replace img tags with amp-img tags and add in the layout type
    $html = str_ireplace( '<img', '<amp-img layout="'.$layout.'"', $html );

    // Add closing tags
    $html = preg_replace( '/<amp-img(.*?)>/', '<amp-img$1></amp-img>', $html );

    return $html;
}

?>

<?php $amped_content = ampImageConverter(get_the_content()); ?>