shell_exec works on the VM, but not via PHP

Question

I have this

public function memory()
{
    $result = [];
    $result['total'] = shell_exec(`free -m | grep Mem: | awk '{ print $2 }'`);
    $result['used'] = shell_exec(`free -m | grep Mem: | awk '{ print $3 }'`);
    $result['free'] = shell_exec(`free -m | grep Mem: | awk '{ print $4 }'`);

    return $result;

}

I am trying to create access my memory of my Ubuntu VM, and graph it.

But API keep return null.

{
total: null,
used: null,
free: null
}

If I run directly in the VM - I got it.

└── free -m
             total       used       free     shared    buffers     cached
Mem:          2002       1244        757         63        159        427
-/+ buffers/cache:        657       1344
Swap:            0          0          0
┌──[john@server]──[/server] 
└── free -m | grep Mem:
Mem:          2002       1244        757         63        159        427                  
┌──[john@server]──[/server] 
└── free -m | grep Mem: | awk '{ print $2 }'                                                
2002
You have new mail in /var/mail/john
┌──[john@server]──[/server] 
└── free -m | grep Mem: | awk '{ print $3 }'                                                
1243
┌──[john@server]──[/server] 
└── free -m | grep Mem: | awk '{ print $4 }'                                                
757

Did I do anything wrong in the way I write my function?

Am I used shell_exec() wrong?

Please give me some suggestions.