Questions about punctuation in PHP for SQL stitching

Problems with punctuation in PHP for SQL stitching
Beginner PHP, encountered a strange problem, let the heroes laughed at, splicing statements as follows:

$sql = ' Select cat_id,cat_name,parent_id from '. $this->table. ' Where parent_id= '. $id;//can pass, positive solution

  $sql = ' Select cat_id,cat_name,parent_id from '. $this->table. ' Where parent_id= '. $id;

Why the vacancy of the symbol, splicing results will be different, puzzled, to seek answers?
------Solution--------------------
You'll know when you compare the output of the two sentences.
The first sentence .... from table where ...
The first sentence .... fromtablewhere ...

In the second sentence, the From, table, and where are connected, can it be correct?

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