PHP Fatal error: Call to undefined function imagettftext()

Why am I getting the error PHP Fatal error: Call to undefined function imagettftext() on line 29?

<?php
ob_start();
session_start();

$strings = '123456789';
$i = 0;
$characters = 6;
$code = '';
while ($i < $characters)
{ 
    $code .= substr($strings, mt_rand(0, strlen($strings)-1), 1);
    $i++;
} 

$_SESSION['captcha'] = $code;

//generate image
$im = imagecreatetruecolor(124, 40);
$foreground = imagecolorallocate($im, 0, 0, 0);
$shadow = imagecolorallocate($im, 173, 172, 168);
$background = imagecolorallocate($im, 255, 255, 255);

imagefilledrectangle($im, 0, 0, 200, 200, $background);

// use your own font!
$font = 'monofont.ttf';

//draw text:
imagettftext($im, 35, 0, 9, 28, $shadow, $font, $code);
imagettftext($im, 35, 0, 2, 32, $foreground, $font, $code);     

// prevent client side  caching
header("Expires: Wed, 1 Jan 1997 00:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

//send image to browser
header ("Content-type: image/png");
imagepng($im);
imagedestroy($im);
?>`

My PHP Info:

According to the PHP manual entry for imagettftext():

This function requires both the GD library and the » FreeType library.

You must be missing one or both of the required libraries in your PHP build.

I solve the same issue on my docker php:7-fpm enviroment, and I post the solution here:

If using `Dockerfile` to setup the enviroment

# more Dockerfile  
FROM php:fpm 
RUN apt-get update && apt-get install -y \
    libfreetype6-dev \
        libmcrypt-dev \
        libpng12-dev \
        libjpeg-dev \
        libpng-dev
    && docker-php-ext-install iconv mcrypt \
    && docker-php-ext-configure gd \
        --enable-gd-native-ttf \
        --with-freetype-dir=/usr/include/freetype2 \
        --with-png-dir=/usr/include \
        --with-jpeg-dir=/usr/include \
    && docker-php-ext-install gd \
    && docker-php-ext-install mbstring \
    && docker-php-ext-enable gd

If want to add the FreeType module on an exist container:

# on docker host machine
docker exec -it $FPM_CONTAINER bash

>>>>

# inside the container
apt-get install -y \
        libfreetype6-dev \
        libmcrypt-dev \
        libpng12-dev \
        libjpeg-dev \
        libpng-dev
docker-php-ext-configure gd \
        --enable-gd-native-ttf \
        --with-freetype-dir=/usr/include/freetype2 \
        --with-png-dir=/usr/include \
        --with-jpeg-dir=/usr/include \
    && docker-php-ext-install gd

exit

<<<<

docker restart $FPM_CONTAINER

Just recompile extension gd.so, under folder php/ext/gd