learning about single and double quotes (.),("), and (')

[wzhetn6puk]

Can you tell me what is the different using \[code\](')single quotes\[/code\] inside \[code\](")quotes\[/code\] and \[code\](")quotes\[/code\] inside \[code\](')single quotes\[/code\]? and at concat, what is the meaning of this \[code\]'".$bla."'\[/code\] I still can not distinguish them.