How to declare functions as variables

Use command substitution:

$ myvar=$(pwd)
$ echo $myvar
/etc
$ echo $(pwd)
/etc

Providing functions as parameters is not intended and will soon get tricky or ugly. The only way I know is via the eval function. Nykakins approach isn't passing a function, but a value, as you can see in this comparing example:

#!/bin/bash
#
# Test providing a function as parameter
#
function f {
  param=$1
  echo "-----------------------"
  date +%T_%N
  sleep 0.3
  echo $param
  date +%T_%N
}

function e {
  param="$1"
  echo "-----------------------"
  date +%T_%N
  sleep 0.3
  eval $param
  date +%T_%N
}

f $(date +%T_%N) 
e "date +%T_%N"

The first style evaluates the function 'date' before passing it's result to the client function f, as you can see, because the second time is before the third:

-----------------------
14:00:12_983387321
14:00:12_980980238
14:00:13_287779378
-----------------------
14:00:13_290126185
14:00:13_594301594
14:00:13_596408013

The second block shows, that the passed over function is evaluated between the two date-calls in the e-function.

Maybe you hadn't that strict naming convention in mind, when asking for a function as variable, but I would say that this makes the difference between a function as variable and the result of a function as variable.

Bash doesn't implement it but otherwise what you are precisely asking for, i.e. use variables exactly as functions, is very simple to implement with ksh93 discipline functions, eg:

#!/bin/ksh93

function pwd.get
{
    .sh.value=$(pwd)
}

cd /var/tmp
echo $pwd
cd /tmp
echo $pwd

When executed, the previous script shows:

/var/tmp
/tmp