Description:
------------
When using a simple addition or minus operator that is prepended by a string and concatenated to it will fail to work properly if the string has a value. When the string is empty (e.g. '' or ' ') it will work properly. The multiplication and division functions operate appropriately.

Reproduce code:
---------------
<?php
$x = 10;
$i = 2;

echo "Addition Test\n";
echo 'Test 1: '. $x + $i . "\n";
echo 'Test 2: '. ($x + $i) .  "\n";
echo 'Test 3: ';
echo $x + $i . "\n";
echo 'Test 4: ';
echo ''. $x + $i ."\n";
echo "\n";

echo "Subtraction Test\n";
echo 'Test 1: '. $x - $i . "\n";
echo 'Test 2: '. ($x - $i) .  "\n";
echo 'Test 3: ';
echo $x - $i . "\n";
echo 'Test 4: ';
echo 'x'. $x - $i ."\n";
echo "\n";


Expected result:
----------------
moffats@dev1:~/public_html/testing> php bug-minus.php
Addition Test
Test 1: 12
Test 2: 12
Test 3: 12
Test 4: 12

Subtraction Test
Test 1: 8
Test 2: 8
Test 3: 8
Test 4: 8



Actual result:
--------------
moffats@dev1:~/public_html/testing> php bug-minus.php
Addition Test
2
Test 2: 12
Test 3: 12
Test 4: 12

Subtraction Test
-2
Test 2: 8
Test 3: 8
Test 4: 8