How to remove newlines when reading files line by line in PHP

Example, PHP reads a file line by line and removes the newline character "n":

//First type:
$content=str_replace("n","",$content);
echo $content;
/ /Or:
$content=str_replace(array("n","r"),"",$content);
//Second type:
$content=preg_replace(" /s/","",$content);
echo $content;
//Third type:
$content=trim($content);