PHP documentation: https://www.php.net/manual/en/
PHP best practices: https://phptherightway.com/
Procedimento, função e método: http://www.serprogramador.com.br/artigos/topico/php/Afinal-qual-a-diferenca-entre-procedimento-funcao-e-metodo-no-PHP
Traversy Media OOP crash course: https://www.youtube.com/watch?v=dQxuYRNbL_M
Object-oriented Programming in 7 minutes | Mosh: https://www.youtube.com/watch?v=pTB0EiLXUC8
Curso em vídeo (história do PHP): https://www.youtube.com/watch?v=F7KzJ7e6EAc&list=PLHz_AreHm4dm4beCCCmW4xwpmLf6EHY9k
Namespaces: https://www.php.net/manual/en/language.namespaces.php