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This is available as Str\splice in the www repository.

Return the string with a slice specified by the offset/length replaced by the given replacement string

namespace HH\Lib\Str;

function splice(
  string $string,
  string $replacement,
  int $offset,
  ?int $length = NULL,
): string;

If the length is omitted or exceeds the upper bound of the string, the remainder of the string will be replaced. If the length is zero, the replacement will be inserted at the offset.

Offset can be positive or negative. When positive, replacement starts from the beginning of the string; when negative, replacement starts from the end of the string.

Some examples:

  • Str\splice("apple", "orange", 0) without $length, $string is replaced, resolving to "orange"
  • Str\splice("apple", "orange", 3) inserting at $offset 3 from the start of $string resolves to "apporange"
  • Str\splice("apple", "orange", -2) inserting at $offset -2 from the end of $string resolves to "apporange"
  • Str\splice("apple", "orange", 0, 0) with $length 0, $replacement is appended at $offset 0 and resolves to "orangeapple"
  • Str\splice("apple", "orange", 5, 0) with $length 0, $replacement is appended at $offset 5 and resolves to "appleorange"

Previously known in PHP as substr_replace.



  • string $string
  • string $replacement
  • int $offset
  • ?int $length = NULL


  • string