$PATH variable is not read

I tried to install MacPorts. I added the following line to my .zshrc :

export PATH=/opt/local/bin/port:$PATH

then when I type the port command I get this :

sudo: port: command not found

But when I try to use MacPorts by calling it directly from its file, it works :

$ /opt/local/bin/port
MacPorts 2.3.4
Entering interactive mode... ("help" for help, "quit" to quit)
[Users/tom] > 

When I type this command echo $PATH this is what I get :

/opt/local/bin/port:/usr/local/mysql/bin:/usr/local/bin:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin

What's wrong how to solve this problem ?

Thank you in advance.

P.S.: I got the same problem when I tried to install git with Homebrew. It only works when I write the whole path (/usr/local/Cellar/git/2.9.3/bin/git) otherwise, despite trying to set the $PATH variable correctly, it uses an older version of git (that was already installed on my computer).

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