PHP interpolating string for function output

The double quote feature in PHP does not evaluate PHP code, it simply replaces variables with their values. If you want to actually evaluate PHP code dynamically (very dangerous), you should use eval:

eval( "function foo() { bar() }" );

Or if you just want to create a function:

$foo = create_function( "", "bar()" );
$foo();

Only use this if there really is no other option.

It is absolutely possible using the string-to-function-name calling technique as Overv's answer indicates. In many trivial substitution cases it reads far better than the alternative syntaxes such as

"<input value='<?php echo 1 + 1 + foo() / bar(); ?>' />"

You need a variable, because the parser expects the $ to be there.

This is where the identity transform works well as a syntactic hack. Just declare an identity function, and assign the name to a variable in scope:

function identity($arg){return $arg;}
$interpolate = "identity";

Then you can pass any valid PHP expression as the function argument:

"<input value='{$interpolate(1 + 1 + foo() / bar() )}' />"

The upside is that you can eliminate a lot of trivial local variables and echo statements.

The downside is that the $interpolate variable falls out of scope, so you would have to repeatedly declare it global inside of functions and methods.