<?php
foreach ($paragraphs as $index => $paragraph) {
    if ( trim( $paragraph ) ) {
        $paragraphs[$index] .= $closing_p;
    }

    $n = $index + 1;
    if ( $n == 3 ) {
        $ad = mp1_func('');
        $paragraphs[$index] .= $ad;
    }

    if ( $n == 10 ) {
        $ad = mp2_func_legacy('');
        $paragraphs[$index] .= $ad;
    }

    if ( $n == 18 ) {
        $ad = img_ad_func('');
        $paragraphs[$index] .= $ad;
    }
}

$a = implode( '', $paragraphs );

echo $a;