How to make contributed module hook execute only if conditions are met. Those functions seem to get automatically invoked. Environment is a custom module.

examples:
1.

    if ($condition) {

       function mymodule_modulename_alter ($parameters) {
         //altering code here
       }
    
    }

2.

        if ($condition) {
               //if condition is met, i call the function included in this custom module
               mymodule_modulename_alter ($parameters)
          }
    
      function mymodule_modulename_alter ($parameters) {
             //but this function will get auto-invoked! I want it run only if $condition is met.
           }
    
    


Thanks