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implode()

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<?php
$string 
implode (
    
// string $glue
    
  // array $pieces
);
?>
Result
$string = 'lastname,email,phone'
Examples
#1 implode  (",",
 [
=> "lastname"=> "email"=> "phone"])
#2 implode  ("hello",
 [])
See also
explode, http_build_query, preg_split