how to get staple value from a function to use it in every were in the code

so basically i got the "date () function" and i want it to record the exact time when the client opens my php page in use that variable in many places in the code without letting the function to set a new value for the variable so if the user spent 24 fucking hour on my website it prints the same value which is the first moment when he sent the fucking first http request to my server so if i created a file with name contains the time with seconds and date and wanted to edit it the fopen function dont give me file not exist error because of the time got changed