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Editorial Team
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Asked: 2026-05-14T19:48:23+00:00

I’m writing a parser in PHP which must be able to handle large in-memory

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I’m writing a parser in PHP which must be able to handle large in-memory strings, so this is a somewhat important issue. (ie, please don’t “premature optimize” flame me, please)

How does the substr function work? Does it make a second copy of the string data in memory, or does it reference the original? Should I worry about calling, for example, $str = substr($str, 1); in a loop?

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1 Answer

  1. Editorial Team

    To further Chad’s comment, your code would require both strings (the full one, and the full-one-minus-first-character) to be in memory at the same time (though not due to the assignment as Chad stated). See:

    $string = str_repeat('x', 1048576);
printf("MEM:  %d\nPEAK: %d\n", memory_get_usage(), memory_get_peak_usage());

substr($string, 1);
printf("MEM:  %d\nPEAK: %d  :-(\n", memory_get_usage(), memory_get_peak_usage());

$string = substr($string, 1);
printf("MEM:  %d\nPEAK: %d  :-(\n", memory_get_usage(), memory_get_peak_usage());

    Outputs something like (memory values are in bytes):

    MEM:  1093256
PEAK: 1093488
MEM:  1093280
PEAK: 2142116  :-(
MEM:  1093276
PEAK: 2142116  :-(
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