There are many string functions in php, but i explain only 
most usable & important string function in PHP are given below:

1. String Function "strlen()":
The "strlen()" function in PHP return the length of the
characters of an array.
Note: strlen() count the character length with white space.

For example:
<?php
echo strlen("SAFCODE By Sir Safder"); 
?>
Output: 21

2. String Function "uuencode()":
The String function "uuencode()" in PHP is used to encodes 
a string using the uuencode algorithm. Its means
uuencode function convert your normal text into
endoded form.

<?php    
	$msg = "Safcode - Code Your Ideas";   
	echo convert_uuencode($msg);
?>
Output:
53!M;W5E(#UY($SM6&MF

3. String Function "uudecode()":
The String function "uudecode()" in PHP is used to decode
a uuencoded string. Its means uudecode function
convert your uuencoded form text into normal text.

<?php    
$msg = "53!M;W5E(#UY($SM6&MF";   
echo convert_uudecode($msg);
?>
Output:
Safcode - Code Your Ideas

4. String Function "implode()":
The implode() function returns a string from the array.
Example:
<?php
$arr = array("HTML", "CSS", "PHP");
echo implode(" ",$arr);
?>
Output:
HTML CSS PHP

5. String Function "md5()":
The md5() function used to calculates the MD5 hash of a string.
The md5() function in PHP uses the RSA Data Security, Inc. 
MD5 Message-Digest Algorithm.
It returns the hash as a 32-character hexadecimal number.
Example:
<?php
$str = "Hello";
echo md5($str);
?>
Output:
8b1a9953c4611296a827abf8c47804d7

6. String Function "strcmp()":
The strcmp() function in PHP compares two values and 
return the answer is 0 or 1, means true or false. 
It is a case sensitive.
Example:
<?php
echo strcmp("Safcode","Safcode");
?>

Output:
0 //its means true

7. String Function "trim()":
The trim() function in PHP erase whitespace and other 
characters from start to end of a string.

<?php
$str = "Hello World!";
echo $str . "<br>";
echo trim($str,"Hed!");
?>

Output:
Hello World!
llo Worl

8. String Function "substr()":
The substr() function in PHP returns a substring of a string. 
Substr() function contains three arguments two are necessary
and last one is optional.

Example1:
<?php
echo substr("Safcode By Sir Safder",11); //starting point 11
?>
Output:
Sir Safder

Example2:
<?php
echo substr("Hello world",0,5) //0 is a starting point and 5 
is a ending point
?>
Output:
Hello

9. String Function "str_shuffle()":
The str_shuffle() function is used to shuffle all the 
character of a string. Its mean str_shuffle() function
move the sequence of string in to randomly order.
Example:
<?php  
  $abc="SAFCODE";  
  echo "Without Shuffle" .abc;
  echo "<br>";  
  echo "After Shuffle" .str_shuffle($abc);
?>
Output:
Without Shuffle: SAFCODE
After Shuffle: OFDSEAC

10. String Function "strtolower()":
The strtolower() function is used to convert string 
into a lowercase.
Example:
<?php
echo strtolower("HELLO WORLD.");
?>
Output:
hello world