How to make multi line to single line based on space character in PHP?

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PHP

string

Example file:

Aiken, L. S., & West, S. G. (1991). Multiple Regression: Testing and Interpreting
        interactions. Sage Publications.
  Bai, X., Yao, W., & Boyer, J. E. (2012). Robust fitting of mixture regression
        models. Computational Statistics & Data Analysis, 56 (7), 2347-2359.
  Bartolucci, F., & Scaccia, L. (2005). The use of mixtures for dealing with
        non-normal regression errors. Computational Statistics & Data Analysis,
        48 (4), 821-834.

If I do

file("example")

,Its considering like below:

1st line- Aiken, L. S., & West, S. G. (1991). Multiple Regression: Testing and Interpreting
              2nd line- interactions. Sage Publications

but it should be in a single line. So, I need to make that two lines as single line based on the space which second line have.As such I need all the reference list to be structured.

If I do like this,

$data = str_replace("\r\n", "", $data); 

then all the lines will become as single line. I want each reference to be single line.

I tried,

$f=file("example")
for($i=0;$i<count($f);$i++){
$f[$i] = str_replace("\r\n", "", $f[$i]);
????
//should replace the new line strings to single line til it finds no space in the
 //new line string so that i can structure second reference in a second line and so
 //on.
}