Pass a fourth parameter to preg_replace( ) to specify the maximum number of replacements you want to make.

 
<?
    $a = "Foo moo boo tool foo";
    $b = preg_replace("/[A-Za-z]oo\b/e", 'strtoupper("$0")', $a, 2);
    print $b;
?>

1.	$0 is set to the whole match, then $1, $2
2.	Converting Line Breaks
3.	preg_replace
4.	preg_replace
5.	preg_replace with index
6.	Pass arrays of regular expressions and replacement strings to preg_replace().
7.	Replacing with preg_replace()
8.	Using Back References with preg_replace()
9.	Using preg_replace() to Replace Patterns
10.	e modifier treats the replacement string in preg_replace() as if it were PHP.
11.	pass arrays as parameter one and parameter two, and preg_replace() performs multiple replaces
12.	Escape a > character in HTML
13.	Replacing URLs with Links
14.	Formatting a Phone Number
15.	Entity encoding matched text
16.	Escaping Special Characters
17.	Regular Expression Replacements
18.	Remove any doubled-up whitespace
19.	Replace any non-space whitespace, with a space
20.	Replace with regular expression
21.	Replacing using backreferences
22.	Quote escaping in backreference replacements

Pass a fourth parameter to preg_replace( ) to specify the maximum number of replacements you want to make. : preg_replace « String « PHP