Description:
------------
Ideally we always need to use spaces between concatenations.

    $a = "Hello" . "World";

But in most cases, we can omit it.

    $a = "Hello"."World";
    $a = $b.$c;

Except when we have two numeric, for obvious reasons.
Speaking softly: it's because it becomes a float.

    $a = 1.1;

However, suppose you want to concatenate a variable with an integer.
What do you would think?

    $a = $b.1;

Exactly! The variable $a will get $b concatenated to the integer 1.
Err... Wait! Wait!

    Parse error: syntax error, unexpected '.1' (T_DNUMBER), expecting ',' or ';'.

This also affects the reverse:

    $a = 1.$b;

The problem occurs because PHP thinks that a dot and a number is a "simplified float" (without integer part, or zero). However, in this case is just (or should be) an integer concatenated, since we can not expect a T_VARIABLE T_LNUMBER side to side.

Test script:
---------------
$a = 1;

// Expect all is true.
var_dump( $a . 1 === "11" );
var_dump( 1 . $a === "11" );

var_dump( $a . .1 === "10.1" );
var_dump( .1 . $a === "0.11" );

var_dump( $a..1 === "10.1" );
var_dump( .1.$a === "0.11" );

// Bad guys.
var_dump( $a.1 === "11" );
var_dump( 1.$a === "11" );

Expected result:
----------------
true
true
true
true
true
true
true
true

Actual result:
--------------
true
true
true
true
true
true
Error
Error