DevEquiv

    PHP substr() Equivalent in Python

    Python's slice syntax (`s[start:stop]`) replaces PHP's substr(), but PHP's third argument is a length while Python's second index is an end position — a common source of off-by-one mistakes when porting.

    Close equivalent

    PHP source
    substr()
    Closest Python API
    slice syntax (str[start:stop])

    Main limitation: PHP's `substr($str, $start, $length)` third argument is a *length* (how many characters to take); Python's `s[start:stop]` second index is an *end position* (exclusive) — `substr($s, 2, 3)` (3 characters starting at index 2) is `s[2:5]` in Python, not `s[2:3]`.

    How to use each function

    Before comparing the two, here's how substr() and slice syntax (str[start:stop]) work on their own.

    PHP: substr()

    substr(string $string, int $offset, ?int $length = null): string

    Extracts a portion of a string starting at $offset (negative counts from the end) and continuing for $length characters (negative trims that many characters off the end instead). Never raises an error for out-of-range offsets — it returns an empty or truncated string instead.

    Returns: The extracted substring.

    PHP 
    substr("hello world", 6, 5); // "world"
substr("hello world", -5); // "world"

    Python: slice syntax (str[start:stop])

    s[start:stop:step]

    Extracts a substring using Python's general slicing syntax. stop is an end position (exclusive), not a length like PHP's substr() third argument — to take n characters starting at start, use s[start:start+n]. Negative indices count from the end, and out-of-range indices never raise an error.

    Returns: The extracted substring.

    Python 
    "hello world"[6:11]  # 'world'
"hello world"[-5:]   # 'world'

    Quick mapping

    PHP substr()
    Closest Python API slice syntax (str[start:stop])

    How it differs

    • PHP's substr($str, $start, $length) third argument is a *length* (how many characters to take); Python's s[start:stop] second index is an *end position* (exclusive) — substr($s, 2, 3) (3 characters starting at index 2) is s[2:5] in Python, not s[2:3].
    • Both languages support negative indices to count from the end of the string, and the negative-index semantics line up well between PHP's substr() and Python's slicing — this is one of the more forgiving parts of the translation.
    • PHP's substr() never throws for out-of-range indices, silently returning an empty string or a truncated result instead; Python's slicing has the exact same forgiving behavior, so this edge case matches cleanly.

    Examples

    Basic substring by length

    PHP 
    substr("hello world", 6, 5);
// "world"
    Python 
    "hello world"[6:11]
# 'world'
    PHP's length (5) becomes Python's end index (6 + 5 = 11) — always add the start and length together to get the Python stop index.

    Negative start (from the end)

    PHP 
    substr("hello world", -5);
// "world"
    Python 
    "hello world"[-5:]
# 'world'
    Negative start indices behave the same way in both languages — take everything from that position to the end.

    Edge cases

    Edge case

    A negative length in PHP's substr($str, $start, $length) means "stop that many characters from the end", not "take |length| characters" — the Python equivalent is s[start:-abs(length)], which is easy to get backwards.

    Edge case

    Passing a start index beyond the string's length returns "" in PHP and '' in Python — both fail gracefully rather than raising an error.

    References

    • PHP Manual — substr — checked 2026-07-29
    • Python docs — Sequence Types — string slicing (v3.x) — checked 2026-07-29

    Last verified 2026-07-29.