amazon web services - PHP on Ubuntu AWS Server: POPEN fails with “sh: : not found”

I am moving my current website over to Amazon AWS from a Red Hat shared server. When I moved my working code, I started to receive an error in my popen call.

The error is: sh: 1: : not found

I found a similar question 5 years ago but there was never really a good answer. popen fails with "sh: : not found"

I use XAMPP on my ubuntu 14.04 laptop for a local testbed and the problem does not arise there, but it does on my AWS instance. Using ubuntu 14.04lts with php5 and apache2 on the instance.

I suspect it might be a PHP setting or path problem but everything I tried has failed, any suggestions?

output = shell_exec('echo abc'); worked fine.

Here is some more info: My Code:

if (file_exists(SitePATH."/solver/dkcube")){

    //echo "Executable is there 
"; 
    error_reporting(E_ALL); 
    $handle = popen("$runvariable 2>&1","r"); 
    echo "Line 1: '$handle'; ".gettype($handle). "
"; 
    $contents = stream_get_contents($handle); 
    echo "Line 2: $contents"; pclose($handle);

The runvariable is basically the dkcube from the file_exists with some pass through variables. The echo for Line 2 is

'Line 2: sh: 1: /var/www/html/solver/dkcube: not found'

Hopefully that gives you some more info.

I tried the sudo -H -u php_user sh -c 'content of runvariable here' and it returned the same error: sh: 1: /var/www/html/solver/dkcube: not found