PHP How can you use string instead of == or != in IF statement

I wonder how i can make if work if i put == or != in variable e.g.

$comparison_operator = " != ";

if ( 5 $comparison_operator 5 ) {
   echo "true";
} else {
   echo "false";
}

i tried with eval() however only works when you put entire expression in it not just variable.

This works

$comparison_operator = " != ";

if ( eval('return(5 $comparison_operator 5);') ) {
   echo "true";
} else {
   echo "false";
}

This does not and i need version that looks like this to work somehow, i tried variable variables without luck as well, any ideas?

$comparison_operator = " != ";

if ( 5 eval('return($comparison_operator)') 5 ) {
   echo "true";
} else {
   echo "false";
}