Warning: session_id(): Cannot change session id when session is active in /var/www/htdocs/captcha.php on line 3

Fatal error: Uncaught Error: Call to undefined function imagettfbbox() in /var/www/lib/vendor/PEAR/Image/Text.php:501 Stack trace: #0 /var/www/lib/vendor/PEAR/Text/CAPTCHA/Driver/Image.php(149): Image_Text->init() #1 /var/www/lib/vendor/PEAR/Text/CAPTCHA.php(114): Text_CAPTCHA_Driver_Image->createCAPTCHA() #2 /var/www/lib/vendor/PEAR/Text/CAPTCHA.php(129): Text_CAPTCHA->generate() #3 /var/www/htdocs/captcha.php(25): Text_CAPTCHA->init(Array) #4 {main} thrown in /var/www/lib/vendor/PEAR/Image/Text.php on line 501