Passing array into in_array()

A slightly abbreviated version, but the same as is being suggested is to return the required data from the function but using array_column() to extract the data...

function get_ship_class()
{
    $csv = array_map("str_getcsv", file("somefile.csv", "r")); 
    $header = array_shift($csv); 

    // Seperate the header from data
    $col = array_search("heavy_shipping_class", $header); 

    // Pass the extracted column back to calling method
    return array_column($csv,$col);
}

And to use it...

if ( in_array( "somevalue", get_ship_class() )) {
   //Process 
}

If you are going to use this returned value a few times, it may be worth storing it in a variable rather than passing it straight into the in_array() method.

This is the answer that the comments are suggesting.

function get_ship_class(){
    $array = array();
    $csv = array_map("str_getcsv", file("somefile.csv", "r")); 
    $header = array_shift($csv); 
    // Seperate the header from data    
    $col = array_search("heavy_shipping_class", $header); 
    foreach ($csv as $row) {      
    array_push($array, $row[$col]);
    // array_push($array, "$row[$col]"); // You may need it as a string instead.
    }
return $array;
}



if( in_array("whatever_you_are_looking_for", get_ship_class()) ){
//code
}