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PHP substr_replace Function

The function substr_replace introduces some additional functionality to compliment str_replace. substr_replace is a more mathematically based replace function, which relies on starting points and lengths to replace parts of strings, as opposed to searching and replacing.

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substr_replace's Four Parameters

There are three required parameters for the substr_replace function (original string, replacement string, starting point) and one that's optional (length).

  1. original string - This is your original string that will be operated on.
  2. replacement string - This string will be used to replace everything in the string from the starting point to the ending point (specified by length).
  3. starting point - This is the place in the original string that will be used to mark the replacement's beginning. A negative value specifies the number of characters from the end of the string.
  4. optional length - How many characters from the original string will be replaced. If no length is specified then the end of the string is used. If a value of 0 is used then no characters will be replaced and an insert is performed. A negative value specifies the number of characters from the end of the string.

substr_replace On Your Mark

This example of substr_replace shows what happens when you omit the length parameter at various starting points.

PHP Code:

//string that needs to be customized
$original = "ABC123 Hello Mr. Cow! DEF321";

//starting point 5
$sp5 = substr_replace($original, "Five", 5);
//starting point 12
$sp12 = substr_replace($original, "Twelve", 12);
//starting point 0
$sp0 = substr_replace($original, "Zero", 0);
//starting point -1
$spneg1 = substr_replace($original, "Negative 1", -1);

//Echo each string
echo "Original String: $original <br />";
echo "Starting Point 5: $sp5 <br />";
echo "Starting Point 12: $sp12 <br />";
echo "Starting Point 0: $sp0 <br />";
echo "Starting Point -1: $spneg1 ";


Original String: ABC123 Hello Mr. Cow! DEF321
Starting Point 5: ABC12Five
Starting Point 12: ABC123 HelloTwelve
Starting Point 0: Zero
Starting Point -1: ABC123 Hello Mr. Cow! DEF32Negative 1

As you can see, when you don't specify the fourth parameter, length, everything after the starting point is replaced by the second parameter replacement string.

Note: The first replacement occurred at position 5, which in $original was the character 3. This 3 and everything onward was replaced with the replacement string. Remember that you start counting character to begin from zero. The $original string could be labeled as so:

  • Letter A - Position 0
  • Letter B - Position 1
  • Letter C - Position 2
  • Letter 1 - Position 3
  • Letter 2 - Position 4
  • Letter 3 - Position 5

substr_replace Specifying a Length

If you want to get any sort of precision out of this function you're going to have to get into the nitty gritty of specifying the exact length of characters you want replaced in your original string.

Imagine that you want to get rid of those ugly pseudo references (ABC123, DEF321) at the beginning and end of the string. Since both of those strings are a length of 6 and we know one is at the very beginning of the string and the other is at the very end of the string we should probably use a starting point of 0 for ABC123 and a value of -6 for DEF321. By having a replacement string of nothing "" we can do something similar to select and delete that we often do in a word processor.

PHP Code:

//string that needs to be customized
$original = "ABC123 Hello Mr. Cow! DEF321";

//remove ABC123 and store in $cleanedstr
$cleanedstr = substr_replace($original, "", 0, 6);
//remove DEF321 from $cleanedstr
$cleanedstr2 = substr_replace($cleanedstr, "", -6, 6);

//Echo each string
echo "Original String: $original <br />";
echo "Clean #1: $cleanedstr <br />";
echo "Clean #2: $cleanedstr2";


Original String: ABC123 Hello Mr. Cow! DEF321
Clean #1: Hello Mr. Cow! DEF321
Clean #2: Hello Mr. Cow!

Make sure that you play around with this function some on your own so you can get a feel for how the starting point and length parameters effect this function.

substr_replace Perform an Insert

By setting the length parameter to zero you can stop substr_replace from removing anything from the original string and just add to it. If we wanted to add a second and third person to our $original string we would want to do this insert operation. Note: instead of counting the characters we've used a couple other PHP functions to figure out the starting positions for us.

PHP Code:

//string that needs to be customized
$original = "Hello Mr. Cow!";

// Get the position of Mr. Cow
$cowpos = strpos($original, "Mr. Cow");

// Find where Mr. Cow ends by adding the length of Mr. Cow
$cowpos_end = $cowpos + strlen("Mr. Cow");

// Insert Mrs. Bear after Mr. Cow
$mrsbear = substr_replace($original, " and Mrs. Bear", $cowpos_end, 0);

// Insert Sensei Shark before Mr. Cow
$senseishark = substr_replace($mrsbear, "Sensei Shark, ", $cowpos, 0);

//Echo each string
echo "Original String: $original <br />";
echo "After Mrs. Bear: $mrsbear <br />";
echo "After Sensei Shark: $senseishark";


Original String: Hello Mr. Cow!
After Mrs. Bear: Hello Mr. Cow and Mrs. Bear!
After Sensei Shark: Hello Sensei Shark, Mr. Cow and Mrs. Bear!

We snuck a new function strlen into that example, but it isn't that complicated of a function, as it stands for "string length."

  • $cowpos_end = $cowpos + strlen("Mr. Cow");

The strlen function takes a string and then counts up how many characters are in it then returns that number. So by calculating the length of "Mr. Cow" and adding that to the position, we find out where the end point is!

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