<?php
function get_chapter($text, $terms) {

    if (empty($text)) return;
    if (empty($terms) || !is_array($terms)) return;

    $values = false;

    $terms_quoted = array();
    foreach ($terms as $term)
        $terms_quoted[] = preg_quote($term, '/');

    // search for matches in $text
    // matches with lowercase, and ignores white spaces...
    if (preg_match('/('.implode('|', $terms_quoted).')\s*(\d+(\.\d+)?)/i', $text, $matches)) {
        if (!empty($matches[2]) && is_numeric($matches[2])) {
            $values = array(
                'term' => $matches[1],
                'value' => $matches[2]
            );
        }
    }

    return $values;
}


$text = '9 text chapter 25.6 text'; // c25.6
$terms = array('chapter', 'chapters');
$chapter = get_chapter($text, $terms);

print_r($chapter);

if ($chapter) {
    echo 'Chapter is: c'. $chapter['value'];
}