This Gist just demonstrates some basic ideas for converting numbers from a numberic format into a word format. It's pretty simplistic (i.e., done in a rush and messy as shit) and is only really intended to provide some basic ideas, and of course, there'll likely be others ways to go about it, too.

ConvertNumber.php

<?php
/**
 * This Gist just demonstrates some basic ideas for converting numbers from a
 * numberic format into a word format. It's pretty simplistic (i.e., done in a
 * rush and messy as shit) and is only really intended to provide some basic
 * ideas, and of course, there'll likely be others ways to go about it, too.
 *
 * Context: https://www.facebook.com/groups/2204685680/permalink/10156073375930681/
 *
 * It will likely need to be expanded upon and adapted to suit your needs (due
 * to that this is just demonstrating some ideas and isn't a complete package,
 * class, etc).
 *
 * Also note: Worth checking out this before using this Gist, in case it suits
 * your needs: http://dk2.php.net/manual/en/class.numberformatter.php
 * (NumberFormatter needs to be installed via PECL before using it, as it isn't
 *  pre-bundled with PHP, but might do the trick, if you're able to use it).
 */

/**
 * Demonstration function "ConvertNumber".
 *
 * @param mixed $Input The original number we're inputting (integer, float,
 *  string, etc, all should be accepted, but if we use strings, we can work
 *  around number precision limitations, so it may be better in some cases).
 * @return string The output.
 */
function ConvertNumber($Input) {
    /** Normalises the input to a float (to get rid of extraneous formatting, etc). */
    $Input = preg_replace('~[^0-9.]~', '', $Input);
    /** Find the character position of the decimal within the input. */
    $Decimal = strpos($Input, '.');

    /** Where we'll eventually dump our output. */
    $Output = '';

    /** Executed if a decimal exists. */
    if ($Decimal !== false) {
        /** The whole number part of the input. */
        $Whole = substr($Input, 0, $Decimal);
        /** The fraction part of the input. */
        $Fraction = substr($Input, $Decimal + 1);
    }

    /** Executed if a decimal DOESN'T exist. */
    else {
        $Whole = $Input;
        $Fraction = '';
    }

    /** So far, so good. Now, what to call for each number? */
    $Numbers = [
        1 => 'one',
        2 => 'two',
        3 => 'three',
        4 => 'four',
        5 => 'five',
        6 => 'six',
        7 => 'seven',
        8 => 'eight',
        9 => 'nine',
        10 => 'ten',
        11 => 'eleven',
        12 => 'twelve',
        13 => 'thirteen',
        14 => 'fourteen',
        15 => 'fifteen',
        16 => 'sixteen',
        17 => 'seventeen',
        18 => 'eighteen',
        19 => 'nineteen',
        20 => 'twenty',
        30 => 'thirty',
        40 => 'fourty',
        50 => 'fifty',
        60 => 'sixty',
        70 => 'seventy',
        80 => 'eighty',
        90 => 'ninety'
    ];

    /**
     * And what to call for each place?
     * See: https://en.wikipedia.org/wiki/Names_of_large_numbers
     */
    $Places = [
        0 => 'zero',
        3 => 'hundred',
        4 => 'thousand',
        7 => 'million',
        10 => 'billion',
        13 => 'trillion',
        16 => 'quadrillion',
        19 => 'quintillion',
        22 => 'sextillion',
        25 => 'septillion',
        28 => 'octillion',
        31 => 'nonillion',
        34 => 'decillion',
        37 => 'undecillion',
        40 => 'duodecillion',
        43 => 'tredecillion',
        46 => 'quattuordecillion',
        49 => 'quindecillion',
        52 => 'sexdecillion',
        55 => 'septendecillion',
        58 => 'octodecillion',
        61 => 'novemdecillion',
        64 => 'vigintillion'
    ];

    /** Misc. other words. */
    $Words = [
        'and' => 'and',
        'point' => 'point'
    ];

    /** In case of numbers 0<x<1. */
    if (empty($Whole)) {
        $Output .= $Places[0] . ' ';
    }

    /** Now, let's iterate through each character of the whole number. */
    for ($Thirds = 0; !empty($Whole); $Whole = substr($Whole, $Thirds + 1)) {
        $Len = strlen($Whole);
        $Thirds = ($Len + 2) % 3;
        $Conjunct = false;
        $Shift = 0;
        if ($Thirds === 2) {
            $Hundred = substr($Whole, 0, 1);
            $Shift++;
            if (isset($Numbers[$Hundred])) {
                $Output .= $Numbers[$Hundred] . ' ';
            }
            $Output .= $Places[3] . ' ';
            $Conjunct = true;
        }
        if ($Thirds === 1 || $Conjunct) {
            $First = substr($Whole, $Shift, 1);
            $Second = substr($Whole, $Shift + 1, 1);
            $Double = $First . $Second;
            if (isset($Numbers[$Double])) {
                if ($Conjunct) {
                    $Output .= $Words['and'] . ' ';
                }
                $Output .= $Numbers[$Double] . ' ';
            } else {
                $First .= '0';
                if (isset($Numbers[$First])) {
                    if ($Conjunct) {
                        $Output .= $Words['and'] . ' ';
                    }
                    $Conjunct = false;
                    $Output .= $Numbers[$First] . ' ';
                }
                if (isset($Numbers[$First])) {
                    if ($Conjunct) {
                        $Output .= $Words['and'] . ' ';
                    }
                    $Output .= $Numbers[$Second] . ' ';
                }
            }
        } elseif ($Thirds === 0) {
            $Number = substr($Whole, $Shift, 1);
            if (isset($Numbers[$Number])) {
                $Output .= $Numbers[$Number] . ' ';
            }
        }
        if ($Len > 3) {
            $Place = ($Len - $Thirds);
            if (isset($Places[$Place])) {
                $Output .= $Places[$Place] . ' ';
            }
        }
    }

    /** Was there a decimal? */
    if ($Decimal !== false) {
        $Output .= $Words['point'] . ' ';
    }

    /** Now, let's iterate through each character of the fraction. */
    while (!empty($Fraction)) {
        $ThisNumber = substr($Fraction, 0, 1);
        if (!$ThisNumber) {
            $Output .= $Places[0] . ' ';
        } elseif (isset($Numbers[$ThisNumber])) {
            $Output .= $Numbers[$ThisNumber] . ' ';
        }
        $Fraction = strlen($Fraction > 1) ? substr($Fraction, 1) : false;
    }

    return ucfirst(trim($Output));
}

echo ConvertNumber(123456789.8765) . "\n";
echo ConvertNumber(12345678.98765) . "\n";
echo ConvertNumber(1234567.898765) . "\n";
echo ConvertNumber(123456.7898765) . "\n";
echo ConvertNumber(12345.67898765) . "\n";
echo ConvertNumber(1234.567898765) . "\n";
echo ConvertNumber(123.4567898765) . "\n";
echo ConvertNumber(12.34567898765) . "\n";
echo ConvertNumber(1.234567898765) . "\n";
echo ConvertNumber(0) . "\n";
echo ConvertNumber(17) . "\n";
echo ConvertNumber(13.111003) . "\n";
echo ConvertNumber(19) . "\n";
echo ConvertNumber(777) . "\n";
echo ConvertNumber('99999999999999999999999999999999999') . "\n";