put variable in sql statement php

This code is work if I change $table to  gel in line 12     how can I change $table to correct way

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$table = "gel";

echo "$table";

$sql ='INSERT INTO pong8_maintable ("timestamp8","Last","TradePrice", "TradeVolume" , "BestBid") 
SELECT 
cast(public.gel."Timestamp" as timestamp) ,

cast(public.$table. "Last" as numeric)  ,

cast(public.gel."Trade Price" as numeric)  ,  
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Error
pg_query(): Query failed: ERROR: syntax error at or near "$" LINE 8: cast(public.$table. "Last" as numeric) , ^
Error
Error while accessing the database:
ERROR: syntax error at or near "$" LINE 8: cast(public.$table. "Last" as numeric) ,